diff --git a/src/datomish/db.cljc b/src/datomish/db.cljc
index 7621928f..a045c504 100644
--- a/src/datomish/db.cljc
+++ b/src/datomish/db.cljc
@@ -495,7 +495,7 @@
   ;; TODO: cache parts.  parts looks like {:db.part/db {:start 0 :current 10}}.  It maps between
   ;; keyword ident part names and integer ranges.
   IDB
-  (query-context [db] (context/->Context (datoms-source db) nil nil))
+  (query-context [db] (context/make-context (datoms-source db)))
 
   (schema [db] (.-schema db))
 
diff --git a/src/datomish/query.cljc b/src/datomish/query.cljc
index 7c27277a..3265e6f2 100644
--- a/src/datomish/query.cljc
+++ b/src/datomish/query.cljc
@@ -40,14 +40,26 @@
 (def sql-quoting-style :ansi)
 
 (defn context->sql-clause [context]
-  (merge
-    {:select (projection/sql-projection context)
+  (let [inner
+        (merge
+          {:select (projection/sql-projection-for-relation context)
 
-     ;; Always SELECT DISTINCT, because Datalog is set-based.
-     ;; TODO: determine from schema analysis whether we can avoid
-     ;; the need to do this.
-     :modifiers [:distinct]}
-    (clauses/cc->partial-subquery (:cc context))))
+           ;; Always SELECT DISTINCT, because Datalog is set-based.
+           ;; TODO: determine from schema analysis whether we can avoid
+           ;; the need to do this.
+           :modifiers [:distinct]}
+          (clauses/cc->partial-subquery (:cc context)))]
+  (if (:has-aggregates? context)
+    (merge
+      (when-not (empty? (:group-by-vars context))
+        ;; We shouldn't need to account for types here, until we account for
+        ;; `:or` clauses that bind from different attributes.
+        {:group-by (map util/var->sql-var (:group-by-vars context))})
+      {:select (projection/sql-projection-for-aggregation context :preag)
+       :modifiers [:distinct]
+       :from [:preag]
+       :with {:preag inner}})
+    inner)))
 
 (defn context->sql-string [context args]
   (->
@@ -56,8 +68,9 @@
     (sql/format args :quoting sql-quoting-style)))
 
 (defn- validate-with [with]
-  (when-not (nil? with)
-    (raise-str "`with` not supported.")))
+  (when-not (or (nil? with)
+                (every? #(instance? Variable %1) with))
+    (raise "Complex :with not supported." {:with with})))
 
 (defn- validate-in [in]
   (when (nil? in)
@@ -92,9 +105,13 @@
     (validate-with with)
     (validate-in in)
     (let [external-bindings (in->bindings in)
-          known-types {}]
+          elements (:elements find)
+          known-types {}
+          group-by-vars (projection/extract-group-by-vars elements with)]
       (assoc context
-             :elements (:elements find)
+             :elements elements
+             :group-by-vars group-by-vars
+             :has-aggregates? (not (nil? group-by-vars))
              :cc (clauses/patterns->cc (:default-source context) where known-types external-bindings)))))
 
 (defn find->sql-clause
@@ -122,7 +139,7 @@
 
 #_
 (datomish.query/find->sql-string
-  (datomish.query.context/->Context (datomish.query.source/datoms-source nil) nil nil)
+  (datomish.query.context/make-context (datomish.query.source/datoms-source nil))
   (datomish.query/parse
     '[:find ?timestampMicros ?page :in $ ?latest :where
       [?page :page/starred true ?t]
@@ -132,7 +149,7 @@
 
 #_
 (datomish.query/find->sql-string
-  (datomish.query.context/->Context (datomish.query.source/datoms-source nil) nil nil)
+  (datomish.query.context/make-context (datomish.query.source/datoms-source nil))
   (datomish.query/parse
     '[:find ?page :in $ ?latest :where
       [?page :page/url "http://example.com/"]
diff --git a/src/datomish/query/context.cljc b/src/datomish/query/context.cljc
index 9437dc5f..b82814ff 100644
--- a/src/datomish/query/context.cljc
+++ b/src/datomish/query/context.cljc
@@ -2,8 +2,18 @@
 ;; License, v. 2.0. If a copy of the MPL was not distributed with this
 ;; file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-;; A context, very simply, holds on to a default source. Eventually
-;; it'll also do projection and similar transforms.
+;; A context, very simply, holds on to a default source and some knowledge
+;; needed for aggregation.
 (ns datomish.query.context)
 
-(defrecord Context [default-source elements cc])
+(defrecord Context
+  [
+   default-source
+   elements        ; The :find list itself.
+   has-aggregates?
+   group-by-vars   ; A list of variables from :find and :with, used to generate GROUP BY.
+   cc              ; The main conjoining clause.
+   ])
+
+(defn make-context [source]
+  (->Context source nil false nil nil))
diff --git a/src/datomish/query/projection.cljc b/src/datomish/query/projection.cljc
index 5f72f2ab..38b1fbb8 100644
--- a/src/datomish/query/projection.cljc
+++ b/src/datomish/query/projection.cljc
@@ -4,20 +4,114 @@
 
 (ns datomish.query.projection
   (:require
+   [honeysql.core :as sql]
    [datomish.query.source :as source]
    [datomish.sqlite-schema :as ss]
-   [datomish.util :as util #?(:cljs :refer-macros :clj :refer) [raise-str cond-let]]
+   [datomish.util :as util #?(:cljs :refer-macros :clj :refer) [raise raise-str cond-let]]
    [datascript.parser :as dp
-    #?@(:cljs [:refer [Pattern DefaultSrc Variable Constant Placeholder]])]
+    #?@(:cljs [:refer [Aggregate Pattern DefaultSrc Variable Constant Placeholder PlainSymbol]])]
    )
-  #?(:clj (:import [datascript.parser Pattern DefaultSrc Variable Constant Placeholder]))
+  #?(:clj (:import [datascript.parser Aggregate Pattern DefaultSrc Variable Constant Placeholder PlainSymbol]))
   )
 
 (defn lookup-variable [cc variable]
   (or (-> cc :bindings variable first)
       (raise-str "Couldn't find variable " variable)))
 
-(defn sql-projection
+(def aggregate-functions
+  {:avg :avg
+   :count :count
+   :max :max
+   :min :min
+   :sum :total
+   })
+
+(defn- aggregate-symbols->projected-var [fn-symbol var-symbol]
+  (keyword (str "_" (name fn-symbol) "_" (subs (name var-symbol) 1))))
+
+(defn- aggregate->projected-var [elem]
+  (aggregate-symbols->projected-var (:symbol (:fn elem))
+                                    (:symbol (first (:args elem)))))
+
+(defn simple-aggregate?
+  "If `elem` is a simple aggregate -- symbolic function, one var arg --
+   return the variable symbol."
+  [elem]
+  (when (instance? Aggregate elem)
+    (let [{:keys [fn args]} elem]
+      (when (and (instance? PlainSymbol fn)
+                 (= 1 (count args)))
+        (let [arg (first args)]
+          (when (instance? Variable arg)
+            (:symbol arg)))))))
+
+(defn- aggregate->var [elem]
+  (when (instance? Aggregate elem)
+    (when-not (simple-aggregate? elem)
+      (raise-str "Only know how to handle simple aggregates."))
+
+    (:symbol (first (:args elem)))))
+
+(defn- variable->var [elem]
+  (when (instance? Variable elem)
+    (:symbol elem)))
+
+(defn- aggregate->projection [elem context lookup-fn]
+  (when (instance? Aggregate elem)
+    (when-not (simple-aggregate? elem)
+      (raise-str "Only know how to handle simple aggregates."))
+
+    (let [var-symbol (:symbol (first (:args elem)))
+          fn-symbol (:symbol (:fn elem))
+          lookup-var (lookup-fn var-symbol)
+          aggregate-fn (get aggregate-functions (keyword fn-symbol))]
+
+      (when-not aggregate-fn
+        (raise-str "Unknown aggregate function " fn-symbol))
+
+      (let [funcall-var (util/aggregate->sql-var aggregate-fn lookup-var)
+            project-as (aggregate-symbols->projected-var fn-symbol var-symbol)]
+        [[funcall-var project-as]]))))
+
+(defn- type-projection
+  "Produce a projection pair by looking up `var` in the provided
+   `extracted-types`."
+  [extracted-types var]
+  (when-let [t (get extracted-types var)]
+    [t (util/var->sql-type-var var)]))
+
+(defn- aggregate-type-projection
+  "Produce a passthrough projection pair for a type field
+   in an inner query."
+  [inner var]
+  (let [type-var (util/var->sql-type-var var)]
+    [(sql/qualify inner type-var) type-var]))
+
+(defn- symbol->projection
+  "Given a variable symbol, produce a projection pair.
+   `lookup-fn` will be used to find a column. For a non-aggregate query,
+   this will typically be a lookup into the CC's bindings. For an
+   aggregate query it'll be a qualification of the same var into the
+   subquery.
+   `known-types` is a type map to decide whether to project a type tag.
+   `type-projection-fn` is like `lookup-fn` but for type tag columns."
+  [var lookup-fn known-types type-projection-fn]
+  (let [lookup-var (lookup-fn var)
+        projected-var (util/var->sql-var var)
+        var-projection [lookup-var projected-var]]
+
+    ;; If the type of a variable isn't explicitly known, we also select
+    ;; its type column so we can transform it.
+    (if-let [type-proj (when (not (contains? known-types var))
+                         (type-projection-fn var))]
+      [var-projection type-proj]
+      [var-projection])))
+
+(defn- variable->projection [elem lookup-fn known-types type-projection-fn]
+  (when (instance? Variable elem)
+    (symbol->projection (:symbol elem) lookup-fn known-types type-projection-fn)))
+
+(defn sql-projection-for-relation
   "Take a `find` clause's `:elements` list and turn it into a SQL
    projection clause, suitable for passing as a `:select` clause to
    honeysql.
@@ -34,68 +128,114 @@
 
      [[:datoms12.e :foo] [:datoms13.e :bar]]
 
+   Note that we also look at `:group-by-vars`, because we need to
+   alias columns and apply `DISTINCT` to those columns in order to
+   aggregate correctly.
+
+   This function unpacks aggregate operations, instead selecting the var.
+
    @param context A Context, containing elements.
    @return a sequence of pairs."
   [context]
-  (let [elements (:elements context)
-        cc (:cc context)
-        known-types (:known-types cc)
-        extracted-types (:extracted-types cc)]
+  (let [{:keys [group-by-vars elements cc]} context
+        {:keys [known-types extracted-types]} cc]
 
-    (when-not (every? #(instance? Variable %1) elements)
-      (raise-str "Unable to :find non-variables."))
+    ;; The primary projections from the :find list.
+    ;; Note that deduplication will be necessary, because we unpack aggregates.
+    (let [projected-vars
+          (map (fn [elem]
+                 (or (aggregate->var elem)
+                     (variable->var elem)
+                     (raise "Only able to :find variables or aggregates."
+                            {:elem elem})))
+               elements)
 
-    ;; If the type of a variable isn't explicitly known, we also select
-    ;; its type column so we can transform it.
+          ;; If we have any GROUP BY requirements from :with, that aren't already
+          ;; included in the above, project them now.
+          additional-vars
+          (clojure.set/difference
+            (set group-by-vars)
+            (set projected-vars))
+
+          full-var-list
+          (distinct (concat projected-vars additional-vars))
+
+          type-proj-fn
+          (partial type-projection extracted-types)
+
+          lookup-fn
+          (partial lookup-variable cc)]
+
+      (mapcat (fn [var]
+                (symbol->projection var lookup-fn known-types type-proj-fn))
+              full-var-list))))
+
+(defn sql-projection-for-aggregation
+  "Project an element list that contains aggregates. This expects a subquery
+   aliased to `inner-table` which itself will project each var with the
+   correct name."
+  [context inner-table]
+  (let [{:keys [group-by-vars elements cc]} context
+        {:keys [known-types extracted-types]} cc
+        lookup-fn (fn [var]
+                    (sql/qualify inner-table (util/var->sql-var var)))
+        type-proj-fn (partial aggregate-type-projection inner-table)]
     (mapcat (fn [elem]
-              (let [var (:symbol elem)
-                    lookup-var (lookup-variable cc var)
-                    projected-var (util/var->sql-var var)
-                    var-projection [lookup-var projected-var]]
-                (if (or (contains? known-types var)
-                        (not (contains? extracted-types var)))
-                  [var-projection]
-                  [var-projection [(get extracted-types var)
-                                   (util/var->sql-type-var var)]])))
-         elements)))
+              (or (variable->projection elem lookup-fn known-types type-proj-fn)
+                  (aggregate->projection elem context lookup-fn)
+                  (raise "Only able to :find variables or aggregates."
+                         {:elem elem})))
+            elements)))
 
 (defn make-projectors-for-columns [elements known-types extracted-types]
   {:pre [(map? extracted-types)
          (map? known-types)]}
-  (map (fn [elem]
-         (let [var (:symbol elem)
-               projected-var (util/var->sql-var var)
-               tag-decoder (memoize
-                             (fn [tag]
-                               (partial ss/<-tagged-SQLite tag)))]
+  (letfn [(variable->projector [elem known-types extracted-types tag-decoder]
+          (when (instance? Variable elem)
+            (let [var (:symbol elem)
+                  projected-var (util/var->sql-var var)]
 
-           (if-let [type (get known-types var)]
-             ;; We know the type! We already know how to decode it.
-             ;; TODO: most of these tags don't actually require calling through to <-tagged-SQLite.
-             ;; TODO: optimize this without making it horrible.
-             (let [decoder (tag-decoder (ss/->tag type))]
-               (fn [row]
-                 (decoder (get row projected-var))))
+              (if-let [type (get known-types var)]
+                ;; We know the type! We already know how to decode it.
+                ;; TODO: most of these tags don't actually require calling through to <-tagged-SQLite.
+                ;; TODO: optimize this without making it horrible.
+                (let [decoder (tag-decoder (ss/->tag type))]
+                  (fn [row]
+                    (decoder (get row projected-var))))
 
-             ;; We don't know the type. Find the type projection column
-             ;; and use it to decode the value.
-             (if (contains? extracted-types var)
-               (let [type-column (util/var->sql-type-var var)]
-                 (fn [row]
-                   (ss/<-tagged-SQLite
-                     (get row type-column)
-                     (get row projected-var))))
+                ;; We don't know the type. Find the type projection column
+                ;; and use it to decode the value.
+                (if (contains? extracted-types var)
+                  (let [type-column (util/var->sql-type-var var)]
+                    (fn [row]
+                      (ss/<-tagged-SQLite
+                        (get row type-column)
+                        (get row projected-var))))
 
-               ;; We didn't extract a type and we don't know it in advance.
-               ;; Just pass through; the :col will look itself up in the row.
-               projected-var))))
-       elements))
+                  ;; We didn't extract a type and we don't know it in advance.
+                  ;; Just pass through; the :col will look itself up in the row.
+                  projected-var)))))
+
+          ;; For now we assume numerics and that everything will shake out in the wash.
+          (aggregate->projector [elem]
+            (when (instance? Aggregate elem)
+              (let [var (aggregate->projected-var elem)]
+                (fn [row]
+                  (get row var)))))]
+
+    (let [tag-decoder (memoize
+                        (fn [tag]
+                          (partial ss/<-tagged-SQLite tag)))]
+      (map (fn [elem]
+             (or (variable->projector elem known-types extracted-types tag-decoder)
+                 (aggregate->projector elem)))
+           elements))))
 
 (defn row-pair-transducer [context]
   (let [{:keys [elements cc]} context
         {:keys [source known-types extracted-types]} cc
 
-        ;; We know the projection will fail above if these aren't simple variables.
+        ;; We know the projection will fail above if these aren't simple variables or aggregates.
         projectors
         (make-projectors-for-columns elements known-types extracted-types)]
 
@@ -104,3 +244,29 @@
         (if err
           [row err]
           [(map (fn [projector] (projector row)) projectors) nil])))))
+
+(defn extract-group-by-vars
+  "Take inputs to :find and, if any aggregates exist in `elements`,
+  return the variable names upon which we should GROUP BY."
+  [elements with]
+  (when (some #(instance? Aggregate %1) elements)
+    (loop [ignore #{}
+           group-by (map :symbol with)
+           e elements]
+
+      (if-let [element (first e)]
+        (if-let [aggregated-var (simple-aggregate? element)]
+          (recur (conj ignore aggregated-var)
+                 group-by
+                 (rest e))
+          (if (instance? Variable element)
+            (let [var (:symbol element)]
+              (recur ignore
+                     (if (contains? ignore var)
+                       group-by
+                       (conj group-by var))
+                     (rest e)))
+            (raise-str "Unknown element." {:element element})))
+
+        ;; Done. Remove any later vars we saw.
+        (remove ignore group-by)))))
diff --git a/src/datomish/util.cljc b/src/datomish/util.cljc
index 8e1da980..8677c2ce 100644
--- a/src/datomish/util.cljc
+++ b/src/datomish/util.cljc
@@ -46,6 +46,11 @@
     (keyword (subs (name x) 1))
     (throw (ex-info (str x " is not a Datalog var.") {}))))
 
+(defn aggregate->sql-var
+  "Turns (:max 'column) into :%max.column."
+  [fn-kw x]
+  (keyword (str "%" (name fn-kw) "." (name x))))
+
 (defn concat-in
   {:static true}
   [m [k & ks] vs]
diff --git a/test/datomish/test/query.cljc b/test/datomish/test/query.cljc
index 77036bbb..b564bdab 100644
--- a/test/datomish/test/query.cljc
+++ b/test/datomish/test/query.cljc
@@ -91,6 +91,24 @@
     :db/valueType :db.type/boolean
     :db/cardinality :db.cardinality/one}])
 
+(def aggregate-schema
+  [{:db/id (d/id-literal :db.part/user)
+    :db.install/_attribute :db.part/db
+    :db/ident :page/url
+    :db/valueType :db.type/string
+    :db/unique :db.unique/identity
+    :db/cardinality :db.cardinality/one}
+   {:db/id (d/id-literal :db.part/user)
+    :db.install/_attribute :db.part/db
+    :db/ident :foo/points
+    :db/valueType :db.type/long
+    :db/cardinality :db.cardinality/many}
+   {:db/id (d/id-literal :db.part/user)
+    :db.install/_attribute :db.part/db
+    :db/ident :foo/visitedAt
+    :db/valueType :db.type/instant
+    :db/cardinality :db.cardinality/many}])
+
 (def schema-with-page
   (concat
     simple-schema
@@ -101,7 +119,7 @@
          :table-alias (comp (make-predictable-gensym) name)))
 
 (defn conn->context [conn]
-  (context/->Context (mock-source (d/db conn)) nil nil))
+  (context/make-context (mock-source (d/db conn))))
 
 (defn- expand [find conn]
   (let [context (conn->context conn)
@@ -443,3 +461,36 @@
              '[:find ?page ?thing :in $ :where
                [?page _ ?thing]]
              conn)))))
+
+(deftest-db test-aggregates conn
+  (let [attrs (<? (<initialize-with-schema conn aggregate-schema))
+        context
+        (populate '[:find ?date (max ?v)
+                    :with ?e
+                    :in $ ?then
+                    :where
+                    [?e :foo/visitedAt ?date]
+                    [(> ?date ?then)]
+                    [?e :foo/points ?v]] conn)]
+
+    (is (= (:group-by-vars context)
+           ['?date '?e]))
+
+    (is (= {:select '([:preag.date :date]
+                      [:%max.preag.v :_max_v])
+            :modifiers [:distinct]
+            :group-by '(:date :e),
+            :with {:preag
+                   {:select '([:datoms0.v :date]
+                               [:datoms1.v :v]
+                               [:datoms0.e :e]),          ; Because we need to group on it.
+                     :modifiers [:distinct],
+                     :from '([:datoms datoms0] [:datoms datoms1]),
+                     :where (list
+                              :and
+                              [:= :datoms0.a (:foo/visitedAt attrs)]
+                              (list :> :datoms0.v (sql/param :then))
+                              [:= :datoms1.a (:foo/points attrs)]
+                              [:= :datoms0.e :datoms1.e])}}
+            :from [:preag]}
+           (query/context->sql-clause context)))))