diff --git a/cpp/ql/lib/semmle/code/cpp/macroflow/MacroFlow.qll b/cpp/ql/lib/semmle/code/cpp/macroflow/MacroFlow.qll
new file mode 100644
index 000000000000..c1285daf3109
--- /dev/null
+++ b/cpp/ql/lib/semmle/code/cpp/macroflow/MacroFlow.qll
@@ -0,0 +1,182 @@
+private import cpp
+
+/**
+ * A module for reasoning about macro expansion in C/C++ code.
+ *
+ * The library allows one to construct path-problem queries that
+ * display paths through nested macro invocations to make
+ * alerts in macro expansions easier to understand.
+ */
+module MacroFlow {
+  /**
+   * Configuration for defining which expressions are interesting for paths from
+   * an inner-most macro expansion to an outer-most macro expansion that results
+   * in an expression.
+   *
+   * For example, to find paths from macro invocations that expand to
+   * `sizeof` expressions with literal arguments, one could do:
+   * ```
+   * module MyConfig implements ConfigSig {
+   *  predicate isSink(Expr e) { e.(SizeofExprOperator).getExprOperand() instanceof Literal }
+   * }
+   *
+   * module Flow = Make<MyConfig>;
+   * import Flow::PathGraph
+   *
+   * from Flow::Node n1, Flow::Node n2, Expr e
+   * where Flow::flowsTo(n1, n2, e)
+   * select e, n1, n2, "Use of sizeof with literal argument."
+   *
+   * ```
+   */
+  signature module ConfigSig {
+    predicate isSink(Expr e);
+  }
+
+  /**
+   * Constructs a successor relation over macro invocations and expressions
+   * based on the given configuration.
+   */
+  module Make<ConfigSig Config> {
+    private predicate rev(MacroInvocation mi) {
+      Config::isSink(mi.getExpr())
+      or
+      exists(MacroInvocation mi1 | rev(mi1) | mi.getParentInvocation() = mi1)
+    }
+
+    private predicate isSource(MacroInvocation mi) {
+      rev(mi) and
+      not exists(MacroInvocation mi0 | rev(mi0) | mi0.getParentInvocation() = mi)
+    }
+
+    private predicate fwd(MacroInvocation mi) {
+      rev(mi) and
+      (
+        isSource(mi)
+        or
+        exists(MacroInvocation mi0 | fwd(mi0) | mi0.getParentInvocation() = mi)
+      )
+    }
+
+    private newtype TNode =
+      TMacroInvocationNode(MacroInvocation mi) { fwd(mi) } or
+      TExprNode(Expr e) {
+        Config::isSink(e) and
+        (
+          exists(MacroInvocation mi |
+            fwd(mi) and
+            mi.getExpr() = e
+          )
+          or
+          // Handle empty paths (i.e., the expression does not have any macros)
+          not exists(MacroInvocation mi | mi.getExpr() = e)
+        )
+      }
+
+    /**
+     * A node in the path graph. A node is either a macro invocation or a sink
+     * expression.
+     */
+    abstract private class NodeImpl extends TNode {
+      /** Gets the macro invocation associated with this node, if any. */
+      abstract MacroInvocation getMacroInvocation();
+
+      /** Gets the expression associated with this node, if any. */
+      abstract Expr getExpr();
+
+      /** Gets the string representation of this node. */
+      abstract string toString();
+
+      /** Gets the location of this node. */
+      abstract Location getLocation();
+    }
+
+    final class Node = NodeImpl;
+
+    private class MacroInvocationNode extends NodeImpl, TMacroInvocationNode {
+      MacroInvocation mi;
+
+      MacroInvocationNode() { this = TMacroInvocationNode(mi) }
+
+      override MacroInvocation getMacroInvocation() { result = mi }
+
+      override Expr getExpr() { none() }
+
+      override Location getLocation() { result = mi.getMacro().getLocation() }
+
+      final override string toString() { result = this.getMacroInvocation().toString() }
+    }
+
+    private class ExprNode extends NodeImpl, TExprNode {
+      Expr e;
+
+      ExprNode() { this = TExprNode(e) }
+
+      override Expr getExpr() { result = e }
+
+      override MacroInvocation getMacroInvocation() {
+        result = any(MacroInvocation mi | mi.getExpr() = e)
+      }
+
+      override Location getLocation() { result = e.getLocation() }
+
+      final override string toString() { result = e.toString() }
+    }
+
+    private predicate steps(Node n1, Node n2) {
+      exists(MacroInvocation mi | n1.(MacroInvocationNode).getMacroInvocation() = mi |
+        mi.getParentInvocation() = n2.(MacroInvocationNode).getMacroInvocation()
+        or
+        not exists(mi.getParentInvocation()) and
+        exists(ExprNode en | en = n2 |
+          mi = en.getMacroInvocation()
+          or
+          // Handle empty paths
+          not exists(en.getMacroInvocation()) and
+          en.getExpr() = mi.getExpr()
+        )
+      )
+    }
+
+    private predicate isSinkNode(Node n) { Config::isSink(n.(ExprNode).getExpr()) }
+
+    private predicate isSourceNode(Node n) {
+      isSource(n.(MacroInvocationNode).getMacroInvocation())
+      or
+      not exists(any(MacroInvocationNode invocation).getMacroInvocation()) and
+      isSinkNode(n)
+    }
+
+    private predicate stepsPlus(Node n1, Node n2) =
+      doublyBoundedFastTC(steps/2, isSourceNode/1, isSinkNode/1)(n1, n2)
+
+    private predicate stepsStar(Node n1, Node n2) {
+      stepsPlus(n1, n2)
+      or
+      // Handle empty paths
+      n1 = n2 and
+      isSourceNode(n1) and
+      isSinkNode(n2)
+    }
+
+    predicate flowsTo(Node n, ExprNode n2, Expr e) {
+      stepsStar(n, n2) and
+      n2.getExpr() = e
+    }
+
+    /**
+     * Provides the query predicates needed to include a graph in a path-problem
+     * query.
+     */
+    module PathGraph {
+      query predicate edges(Node n1, Node n2) {
+        steps(n1, n2)
+        or
+        // Handle empty paths
+        isSourceNode(n1) and
+        isSinkNode(n2) and
+        n1 = n2
+      }
+    }
+  }
+}