This talk explores the evolution of gradual typing from the perspective that the best end state for a large codebase is full static types. Hack’s Sound Dynamic is designed to bridge the gap for a large codebase where a “big bang” adoption of static typing is infeasible. Hack evolved from PHP. The Hack type system and tooling are such that “’types don’t lie”: users know which types are (un)trustworthy and have mechanisms for gradually improving type safety of the codebase.