diff --git a/src/v3/core/__snapshots__/address.test.js.snap b/src/v3/core/__snapshots__/address.test.js.snap
new file mode 100644
index 0000000..268a64b
--- /dev/null
+++ b/src/v3/core/__snapshots__/address.test.js.snap
@@ -0,0 +1,3 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`core/address makeAddressModule has a stable internal representation 1`] = `"\\"F\\\\u0000\\\\u0000hello\\\\u0000\\\\u0000\\\\u0000sweet\\\\u0000world\\\\u0000\\\\u0000\\\\u0000\\\\u0000\\""`;
diff --git a/src/v3/core/address.js b/src/v3/core/address.js
index 9069df1..598f624 100644
--- a/src/v3/core/address.js
+++ b/src/v3/core/address.js
@@ -115,29 +115,79 @@ export function makeAddressModule(options: Options): AddressModule<string> {
     otherNoncesWithSeparators.set(otherNonce + separator, otherName);
   }
 
-  const _ = {name, nonceWithSeparator};
-
   function assertValid(address: Address, what?: string): void {
-    const _ = {address, what};
-    throw new Error("assertValid");
+    // TODO(perf): If this function becomes a bottleneck, consider
+    // omitting it entirely in production. If this is undesirable, a
+    // number of micro-optimizations can be made.
+    const prefix = what == null ? "" : `${what}: `;
+    if (address == null) {
+      throw new Error(prefix + `expected ${name}, got: ${String(address)}`);
+    }
+    if (!address.endsWith(separator)) {
+      throw new Error(prefix + `expected ${name}, got: ${stringify(address)}`);
+    }
+    if (!address.startsWith(nonceWithSeparator)) {
+      for (const [
+        otherNonceWithSeparator,
+        otherName,
+      ] of otherNoncesWithSeparators) {
+        if (address.startsWith(otherNonceWithSeparator)) {
+          throw new Error(
+            prefix + `expected ${name}, got ${otherName}: ${stringify(address)}`
+          );
+        }
+      }
+      throw new Error(prefix + `expected ${name}, got: ${stringify(address)}`);
+    }
+  }
+
+  function partsString(parts: $ReadOnlyArray<string>): string {
+    // This is needed to properly print arrays containing `undefined`.
+    return "[" + parts.map((p) => String(stringify(p))).join(",") + "]";
   }
 
   function assertValidParts(
     parts: $ReadOnlyArray<string>,
     what?: string
   ): void {
-    const _ = {parts, what};
-    throw new Error("assertValidParts");
+    // TODO(perf): If this function becomes a bottleneck, consider
+    // omitting it entirely in production. If this is undesirable, a
+    // number of micro-optimizations can be made.
+    const prefix = what == null ? "" : `${what}: `;
+    if (parts == null) {
+      throw new Error(
+        prefix + `expected array of parts, got: ${String(parts)}`
+      );
+    }
+    parts.forEach((s: string) => {
+      if (s == null) {
+        throw new Error(
+          prefix +
+            `expected array of parts, got ${String(s)} in: ${partsString(
+              parts
+            )}`
+        );
+      }
+      if (s.indexOf(separator) !== -1) {
+        const where = `${stringify(s)} in ${partsString(parts)}`;
+        throw new Error(prefix + `part contains NUL character: ${where}`);
+      }
+    });
+  }
+
+  function nullDelimited(components: $ReadOnlyArray<string>): string {
+    return [...components, ""].join(separator);
   }
 
   function fromParts(parts: $ReadOnlyArray<string>): Address {
-    const _ = parts;
-    throw new Error("fromParts");
+    assertValidParts(parts);
+    return nonce + separator + nullDelimited(parts);
   }
 
   function toParts(address: Address): string[] {
-    const _ = address;
-    throw new Error("toParts");
+    assertValid(address);
+    const parts = address.split(separator);
+    return parts.slice(1, parts.length - 1);
   }
 
   function toString(address: Address): string {
diff --git a/src/v3/core/address.test.js b/src/v3/core/address.test.js
index 7d194ce..25f3dbe 100644
--- a/src/v3/core/address.test.js
+++ b/src/v3/core/address.test.js
@@ -58,5 +58,163 @@ describe("core/address", () => {
         FooAddress.assertValid = FooAddress.assertValid;
       }).toThrow(/read.only property/);
     });
+
+    it("has a stable internal representation", () => {
+      const input = ["", "hello", "", "", "sweet", "world", "", "", ""];
+      const address = makeModules().FooAddress.fromParts(input);
+      // We stringify the address here because otherwise literal NUL
+      // characters will appear in the snapshot file.
+      expect(JSON.stringify(address)).toMatchSnapshot();
+    });
+
+    describe("assertValid", () => {
+      const {FooAddress, BarAddress, WatAddress} = makeModules();
+      it("rejects `undefined`", () => {
+        expect(() => {
+          // $ExpectFlowError
+          FooAddress.assertValid(undefined, "widget");
+        }).toThrow("widget: expected FooAddress, got: undefined");
+      });
+      it("rejects `null`", () => {
+        expect(() => {
+          // $ExpectFlowError
+          FooAddress.assertValid(null, "widget");
+        }).toThrow("widget: expected FooAddress, got: null");
+      });
+      it("rejects an address from a known module", () => {
+        const bar = BarAddress.fromParts(["hello"]);
+        expect(() => {
+          FooAddress.assertValid(bar, "widget");
+        }).toThrow("widget: expected FooAddress, got BarAddress:");
+      });
+      it("rejects an address from an unknown module", () => {
+        const wat = WatAddress.fromParts(["hello"]);
+        expect(() => {
+          FooAddress.assertValid(wat, "widget");
+        }).toThrow("widget: expected FooAddress, got:");
+      });
+      it("rejects a string that starts with the nonce but no separator", () => {
+        expect(() => {
+          FooAddress.assertValid("F/things\0", "widget");
+        }).toThrow("widget: expected FooAddress, got:");
+      });
+      it("rejects a string that does not end with a separator", () => {
+        const address = FooAddress.fromParts(["hello"]);
+        const truncated = address.substring(0, address.length - 2);
+        expect(() => {
+          FooAddress.assertValid(truncated, "widget");
+        }).toThrow("widget: expected FooAddress, got:");
+      });
+      it("rejects junk", () => {
+        expect(() => {
+          FooAddress.assertValid("junk", "widget");
+        }).toThrow("widget: expected FooAddress, got:");
+      });
+    });
+
+    describe("assertValidParts", () => {
+      const {FooAddress} = makeModules();
+      it("rejects `undefined`", () => {
+        expect(() => {
+          // $ExpectFlowError
+          FooAddress.assertValidParts(undefined, "widget");
+        }).toThrow("widget: expected array of parts, got: undefined");
+      });
+      it("rejects `null`", () => {
+        expect(() => {
+          // $ExpectFlowError
+          FooAddress.assertValidParts(null, "widget");
+        }).toThrow("widget: expected array of parts, got: null");
+      });
+      it("rejects an array containing `undefined`", () => {
+        expect(() => {
+          // $ExpectFlowError
+          FooAddress.assertValidParts(["hello", undefined, "world"], "widget");
+        }).toThrow(
+          "widget: expected array of parts, got undefined in: " +
+            '["hello",undefined,"world"]'
+        );
+      });
+      it("rejects an array containing `null`", () => {
+        expect(() => {
+          // $ExpectFlowError
+          FooAddress.assertValidParts(["hello", null, "world"], "widget");
+        }).toThrow(
+          "widget: expected array of parts, got null in: " +
+            '["hello",null,"world"]'
+        );
+      });
+      it("rejects an array with a string containing a NUL character", () => {
+        expect(() => {
+          FooAddress.assertValidParts(["hello", "n\0o", "world"], "widget");
+        }).toThrow(
+          "widget: part contains NUL character: " +
+            '"n\\u0000o" in ["hello","n\\u0000o","world"]'
+        );
+      });
+    });
+
+    describe("fromParts", () => {
+      const {FooAddress, BarAddress} = makeModules();
+
+      // We use this next test as a proxy for fully correct validation,
+      // in conjunction with tests on `assertValid` and
+      // `assertValidParts`.
+      it("validates parts", () => {
+        expect(() => {
+          // $ExpectFlowError
+          FooAddress.fromParts(["hello", null, "world"]);
+        }).toThrow(
+          'expected array of parts, got null in: ["hello",null,"world"]'
+        );
+      });
+
+      it("encodes the address kind for the empty address", () => {
+        const foo = FooAddress.fromParts([]);
+        const bar = BarAddress.fromParts([]);
+        expect(foo).not.toEqual(bar);
+      });
+      it("encodes the address kind for a normal address", () => {
+        const foo = FooAddress.fromParts(["hello", "world"]);
+        const bar = BarAddress.fromParts(["hello", "world"]);
+        expect(foo).not.toEqual(bar);
+      });
+    });
+
+    describe("toParts", () => {
+      const {FooAddress, BarAddress} = makeModules();
+
+      // We use this next test as a proxy for fully correct validation,
+      // in conjunction with tests on `assertValid`.
+      it("validates address kind", () => {
+        const bar = BarAddress.fromParts(["hello"]);
+        expect(() => {
+          FooAddress.toParts(bar);
+        }).toThrow("expected FooAddress, got BarAddress:");
+      });
+
+      describe("is a left identity for `fromParts`", () => {
+        function check(description, inputParts) {
+          it(description, () => {
+            const address = FooAddress.fromParts(inputParts);
+            const parts = FooAddress.toParts(address);
+            expect(parts).toEqual(inputParts);
+          });
+        }
+        check("on the empty input", []);
+        check("on an input made of one empty part", [""]);
+        check("on an input made of lots of empty parts", ["", "", ""]);
+        check("on an input with lots of empty parts throughout", [
+          ...["", ""],
+          "hello",
+          ...["", "", ""],
+          "sweet",
+          "world",
+          ...["", "", "", ""],
+        ]);
+        check("on a singleton input", ["jar"]);
+        check("on an input with repeated components", ["jar", "jar"]);
+      });
+    });
   });
 });