diff --git a/ConNF/Setup/Small.lean b/ConNF/Setup/Small.lean
new file mode 100644
index 0000000000..b6e9cb2926
--- /dev/null
+++ b/ConNF/Setup/Small.lean
@@ -0,0 +1,81 @@
+import ConNF.Setup.Params
+
+/-!
+# Smallness
+
+In this file, we define the notion of a small set, and prove many of the basic properties of small
+sets.
+
+## Main declarations
+
+* `ConNF.Small`: A set is *small* if its cardinality is less than `#κ`.
+-/
+
+noncomputable section
+universe u
+
+open Cardinal Set
+open scoped symmDiff
+
+namespace ConNF
+
+variable [Params.{u}] {ι α β : Type _} {s t u : Set α}
+
+/-- A set is *small* if its cardinality is less than `#κ`. -/
+def Small (s : Set α) : Prop :=
+  #s < #κ
+
+theorem Small.lt : Small s → #s < #κ :=
+  id
+
+/-!
+## Criteria for smallness
+-/
+
+theorem small_of_subsingleton (h : s.Subsingleton) : Small s :=
+  h.cardinal_mk_le_one.trans_lt <| one_lt_aleph0.trans_le aleph0_lt_κ.le
+
+@[simp]
+theorem small_empty : Small (∅ : Set α) :=
+  small_of_subsingleton subsingleton_empty
+
+@[simp]
+theorem small_singleton (x : α) : Small {x} :=
+  small_of_subsingleton subsingleton_singleton
+
+/-- Subsets of small sets are small. We say that the 'smallness' relation is monotone. -/
+theorem Small.mono (h : s ⊆ t) : Small t → Small s :=
+  (mk_le_mk_of_subset h).trans_lt
+
+theorem Small.congr (h : s = t) : Small t → Small s :=
+  Small.mono (subset_of_eq h)
+
+/-- Unions of small subsets are small. -/
+theorem small_union (hs : Small s) (ht : Small t) : Small (s ∪ t) :=
+  (mk_union_le _ _).trans_lt <| add_lt_of_lt Params.κ_isRegular.aleph0_le hs ht
+
+/-- Unions of small subsets are small. -/
+theorem small_symmDiff (hs : Small s) (ht : Small t) : Small (s ∆ t) :=
+  (small_union hs ht).mono symmDiff_subset_union
+
+theorem small_iUnion (hι : #ι < #κ) {f : ι → Set α} (hf : ∀ i, Small (f i)) :
+    Small (⋃ i, f i) :=
+  (mk_iUnion_le _).trans_lt <|
+    mul_lt_of_lt Params.κ_isRegular.aleph0_le hι <| iSup_lt_of_isRegular Params.κ_isRegular hι hf
+
+theorem small_iUnion_Prop {p : Prop} {f : p → Set α} (hf : ∀ i, Small (f i)) : Small (⋃ i, f i) :=
+  by by_cases p <;> simp_all
+
+theorem small_biUnion {s : Set ι} (hs : Small s) {f : ∀ i ∈ s, Set α}
+    (hf : ∀ (i : ι) (hi : i ∈ s), Small (f i hi)) : Small (⋃ (i : ι) (hi : i ∈ s), f i hi) :=
+  (small_iUnion hs (λ i ↦ hf i i.prop)).congr (Set.biUnion_eq_iUnion _ _)
+
+/-- The image of a small set under any function `f` is small. -/
+theorem Small.image (f : α → β) : Small s → Small (f '' s) :=
+  mk_image_le.trans_lt
+
+/-- The preimage of a small set under an injective function `f` is small. -/
+theorem Small.preimage (f : β → α) (h : f.Injective) : Small s → Small (f ⁻¹' s) :=
+  (mk_preimage_of_injective f s h).trans_lt
+
+end ConNF
diff --git a/blueprint/src/chapters/environment.tex b/blueprint/src/chapters/environment.tex
index c62fbd20a2..56cd5c8d22 100644
--- a/blueprint/src/chapters/environment.tex
+++ b/blueprint/src/chapters/environment.tex
@@ -54,6 +54,8 @@ \section{Model parameters}
 \begin{definition}[small]
   \label{def:Small}
   \uses{def:Params}
+  \lean{ConNF.Small}
+  \leanok
   A set \( s : \Set(\tau) \) is called \emph{small} if \( \#s < \#\kappa \).
   Smallness is stable under subset, intersection, small-indexed unions, symmetric difference, direct image, injective preimage, and many other operations (each of which should be its own lemma when formalised).
   Sets \( s, t : \Set(\tau) \) are called \emph{near} if \( s \symmdiff t \) is small; in this case, we write \( s \near t \).