diff --git a/crates/frontend/src/resolve.rs b/crates/frontend/src/resolve.rs
index 36f6ac7..13eaae4 100644
--- a/crates/frontend/src/resolve.rs
+++ b/crates/frontend/src/resolve.rs
@@ -22,7 +22,9 @@
 use std::{collections::BTreeMap, convert::Infallible, marker::PhantomData, sync::Arc};
 
 use indexmap::IndexSet;
+use kerolox_ir::EnumRepr;
 use salsa::Update;
+use strum::EnumIter;
 
 use crate::prelude::*;
 
@@ -206,7 +208,8 @@ pub fn expr<'db>(
                 relation: loaded.insert_full(relation_name(db, apply.head.clone())).0,
             }))),
 
-            ast::ExprExtra::Aggregate(aggregate) => todo!(),
+            // aggregate resolution validates the specific operation
+            ast::ExprExtra::Aggregate(agg) => todo!(),
         },
 
         // TODO: handle unreachability better?
@@ -322,3 +325,88 @@ pub struct Apply<'db> {
     /// The parameters to the function or query.
     pub body: Expr<'db>,
 }
+
+/// A resolved aggregate operator.
+#[derive(Clone, PartialEq, Eq, Hash)]
+pub struct Aggregate<'db> {
+    /// The resolved kind of aggregate operator.
+    pub kind: AggregateKind,
+
+    /// The list of witnesses (variable indices) to the body.
+    pub witnesses: Vec<WithAst<usize>>,
+
+    /// The body of the aggregate.
+    pub body: AggregateBody<'db>,
+}
+
+/// A resolved aggregate operator kind.
+#[derive(Clone, PartialEq, Eq, Hash)]
+pub enum AggregateKind {
+    /// A "bound" aggregate: selects a variable to perform the aggregate over.
+    Bound {
+        /// The kind of aggregate operator.
+        kind: WithAst<BoundAggregateKind>,
+
+        /// The index of the variable to bind over.
+        variable: WithAst<usize>,
+    },
+
+    /// An "unbound" aggregate: no variable needs to be selected to perform the operation.
+    Unbound {
+        /// The kind of aggregate operator.
+        kind: WithAst<UnboundAggregateKind>,
+    },
+
+    /// The aggregate name could not be resolved.
+    Unknown(WithAst<String>),
+}
+
+/// A bound aggregate operator.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, EnumIter)]
+pub enum BoundAggregateKind {
+    Sum,
+    Average,
+    Min,
+    Max,
+}
+
+impl EnumRepr for BoundAggregateKind {
+    fn repr(&self) -> &'static str {
+        use BoundAggregateKind::*;
+        match self {
+            Sum => "sum",
+            Average => "average",
+            Min => "min",
+            Max => "max",
+        }
+    }
+}
+
+/// An unbound aggregate operator.
+#[derive(Copy, Clone, PartialEq, Eq, Hash, EnumIter)]
+pub enum UnboundAggregateKind {
+    Count,
+    Any,
+    All,
+}
+
+impl EnumRepr for UnboundAggregateKind {
+    fn repr(&self) -> &'static str {
+        use UnboundAggregateKind::*;
+        match self {
+            Count => "count",
+            Any => "any",
+            All => "all",
+        }
+    }
+}
+
+/// An aggregate operator's body.
+#[derive(Clone, PartialEq, Eq, Hash)]
+pub enum AggregateBody<'db> {
+    /// A single application operation.
+    Apply(Apply<'db>),
+
+    /// A list of clauses, like in a [RuleBody].
+    Body(Vec<Expr<'db>>),
+}