diff --git a/nullability/inference/merge.h b/nullability/inference/merge.h
new file mode 100644
index 0000000..e4e606d
--- /dev/null
+++ b/nullability/inference/merge.h
@@ -0,0 +1,54 @@
+// Part of the Crubit project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// Combines evidence of the nullability of a symbol into inference conclusions.
+//
+// This evidence was gathered from local analysis across the codebase in a
+// "map" phase, merging this evidence is conceptually the "reduce" phase.
+//
+// To allow this to be distributed efficiently for commonly-used symbols,
+// merging is performed incrementally by repeatedly combining partial results.
+
+#ifndef CRUBIT_NULLABILITY_INFERENCE_MERGE_H_
+#define CRUBIT_NULLABILITY_INFERENCE_MERGE_H_
+
+#include <vector>
+
+#include "nullability/inference/inference.proto.h"
+#include "llvm/ADT/ArrayRef.h"
+
+namespace clang::tidy::nullability {
+
+// Build a Partial representing a single piece of evidence.
+Partial partialFromEvidence(const Evidence &);
+// Update LHS to include the evidence from RHS.
+// The two must describe the same symbol.
+// The merging of partials is commutative and associative.
+void mergePartials(Partial &LHS, const Partial &RHS);
+// Form nullability conclusions from a set of evidence.
+Inference finalize(const Partial &);
+
+struct InferResult {
+  Inference::Nullability Nullability;
+  bool Conflict = false;
+};
+// Final inference decisions, based on event counts.
+// TODO: once this interface sticks, move to a dedicated file.
+InferResult infer(llvm::ArrayRef<unsigned> EventCounts);
+
+// Combines local evidence about symbol nullability to form a global conclusion.
+// All evidence must for be the same symbol, and there must be some.
+//
+// This signature fundamentally limits the scalability of merging: we must see
+// all the evidence for a symbol at once.
+inline Inference mergeEvidence(llvm::ArrayRef<Evidence> Ev) {
+  Partial P = partialFromEvidence(Ev.front());
+  for (const auto &E : Ev.drop_front())
+    mergePartials(P, partialFromEvidence(E));
+  return finalize(P);
+}
+
+}  // namespace clang::tidy::nullability
+
+#endif