diff --git a/Anoma/Base.juvix b/Anoma/Base.juvix
index 4b09628..e9621ac 100644
--- a/Anoma/Base.juvix
+++ b/Anoma/Base.juvix
@@ -26,6 +26,8 @@ import Stdlib.Prelude open;
 
 --- A module containing types that are currently unknown or require furhter discussion.
 -- SPEC_QUESTION / INST_QUESTION: What should these types be?
+import Anoma.Base.Set as Set open using {Set} public;
+
 module Unknown;
   -- This is currently equal to the function type in RM instantiation.
   axiom ResourceLogicFieldType : Type;
@@ -78,10 +80,6 @@ module Opaque;
   --- The zero value of `Opaque.Delta`.
   -- This is not in the specification but is used to initialize delta fields.
   axiom zeroDelta : Delta;
-
-  -- TODO: Other Set functions will be required.
-  -- See https://github.com/anoma/nspec/issues/197
-  axiom Set : Type -> Type;
 end;
 
 -- Add opaque types to the scope for convenience.
diff --git a/Anoma/Base/Set.juvix b/Anoma/Base/Set.juvix
new file mode 100644
index 0000000..1c91231
--- /dev/null
+++ b/Anoma/Base/Set.juvix
@@ -0,0 +1,147 @@
+{-- This module contains the Set API to be used when interacting with Anoma.
+
+It is recommended that you import this module as follows:
+
+import Anoma.Base.Set as Set open using {Set}
+
+or
+
+import Anoma.Base.Set as AnomaSet open using {Set as AnomaSet}
+
+if you use this module and the standard library set from Stdlib.Data.Set in the
+same module. --}
+module Anoma.Base.Set;
+
+import Stdlib.Prelude open hiding {foldl; all; any; isMember; for};
+import Stdlib.Data.Set as StdlibSet open using {Set as StdlibSet};
+
+--- A module containing the builtin Anoma Set API.
+module Internal;
+
+  axiom Set : Type -> Type;
+
+  axiom lookup : {A : Type} -> (elem : A) -> (set : Set A) -> Maybe A;
+
+  axiom isMember : {A : Type} -> (elem : A) -> (set : Set A) -> Bool;
+
+  axiom insert : {A : Type} -> (elem : A) -> (set : Set A) -> Set A;
+
+  axiom toList : {A : Type} -> (set : Set A) -> List A;
+
+  axiom fromList : {A : Type} -> (list : List A) -> Set A;
+
+  axiom size : {A : Type} -> (set : Set A) -> Nat;
+
+  axiom delete : {A : Type} -> (elem : A) -> (set : Set A) -> Set A;
+
+  axiom map : {A B : Type} -> (fun : A -> B) -> (set : Set A) -> Set B;
+
+  --- Implemented using `++rep:in` https://developers.urbit.org/reference/hoon/stdlib/2h#repin
+  -- NB: The initial accumulator should be the second default argument of the function.
+  axiom foldl : {A B : Type} -> (fun : B -> A -> B) -> (acc : B) -> Set A -> B;
+
+  axiom union : {A : Type} -> (set1 set2 : Set A) -> Set A;
+
+  axiom intersection : {A : Type} -> (set1 set2 : Set A) -> Set A;
+
+  axiom difference : {A : Type} -> (set1 set2 : Set A) -> Set A;
+
+  axiom all : {A : Type} -> (predicate : A -> Bool) -> (set : Set A) -> Bool;
+
+  axiom any : {A : Type} -> (predicate : A -> Bool) -> (set : Set A) -> Bool;
+end;
+
+open Internal using {Set} public;
+
+{-# inline: true #-}
+singleton {A} (elem : A) : Set A := Internal.fromList {A} [elem];
+
+{-# inline: true #-}
+empty {A} : Set A := Internal.fromList {A} [];
+
+{-# inline: true #-}
+lookup {A} (elem : A) (set : Set A) : Maybe A := Internal.lookup {A} elem set;
+
+{-# inline: true #-}
+isMember {A} (elem : A) (set : Set A) : Bool := Internal.isMember {A} elem set;
+
+{-# inline: true #-}
+insert {A} (elem : A) (set : Set A) : Set A := Internal.insert {A} elem set;
+
+{-# inline: true #-}
+toList {A} (set : Set A) : List A := Internal.toList {A} set;
+
+{-# inline: true #-}
+fromList {A} (list : List A) : Set A := Internal.fromList {A} list;
+
+fromStdlibSet {A} (set : StdlibSet A) : Set A :=
+  set |> StdlibSet.toList |> fromList;
+
+toStdlibSet {A} {{Ord A}} (set : Set A) : StdlibSet A :=
+  set |> toList |> StdlibSet.fromList;
+
+{-# inline: true #-}
+size {A} (set : Set A) : Nat := Internal.size {A} set;
+
+{-# inline: true #-}
+delete {A} (elem : A) (set : Set A) : Set A := Internal.delete elem set;
+
+{-# inline: true #-}
+union {A} (set1 set2 : Set A) : Set A := Internal.union {A} set1 set2;
+
+{-# inline: true #-}
+intersection {A} (set1 set2 : Set A) : Set A := Internal.intersection set1 set2;
+
+{-# inline: true #-}
+difference {A} (set1 set2 : Set A) : Set A := Internal.difference set1 set2;
+
+{-# inline: true #-}
+isEmpty {A} (set : Set A) : Bool := size set == 0;
+
+syntax iterator map {init := 0; range := 1};
+{-# inline: true #-}
+map {A B} (fun : A -> B) (set : Set A) : Set B := Internal.map {A} {B} fun set;
+
+{-# inline: true #-}
+foldl {A B} (fun : B -> A -> B) (acc : B) (set : Set A) : B :=
+  Internal.foldl {A} {B} fun acc set;
+
+-- The hoon stdlib does not define foldr so we cannot implement Foldable.
+syntax iterator for {init := 1; range := 1};
+{-# inline: true #-}
+for : {A B : Type} -> (B -> A -> B) -> B -> Set A -> B := foldl;
+
+syntax iterator all {init := 0; range := 1};
+{-# inline: true #-}
+all {A} (predicate : A -> Bool) (set : Set A) : Bool :=
+  Internal.all {A} predicate set;
+
+syntax iterator any {init := 0; range := 1};
+{-# inline: true #-}
+any {A} (predicate : A -> Bool) (set : Set A) : Bool :=
+  Internal.any {A} predicate set;
+
+isSubset {A} (set1 set2 : Set A) : Bool :=
+  all (x in set1) {
+    isMember x set2
+  };
+
+syntax iterator filter {init := 0; range := 1};
+{-# specialize: [1] #-}
+filter {A} (predicate : A -> Bool) (set : Set A) : Set A :=
+  for (acc := empty) (x in set) {
+    if
+      | predicate x := insert x acc
+      | else := acc
+  };
+
+{-# specialize: [1] #-}
+disjointUnion {A} (set1 set2 : Set A) : Result A (Set A) :=
+  for (acc := ok set2) (x in set1) {
+    case acc of
+      | error _ := acc
+      | ok set :=
+        if
+          | isMember x set2 := error x
+          | else := ok (insert x set)
+  };