Employers routinely assume a paycheck buys whatever their employees invent, and routinely learn otherwise at a financing, an acquisition, or the courthouse door. This article explains how to draft employee invention assignment agreements that survive diligence and litigation, from the default rule that inventors own inventions through the shop-right and hired-to-invent doctrines, the decisive gap between a present assignment and a mere promise to assign that sank Stanford's case in Stanford v. Roche, and the FilmTec line that turned "hereby assigns" into words of conveyance. It maps the state carve-out statutes that void overreaching clauses, the consideration problem that quietly defeats mid-employment rollouts, the definitional and cooperation machinery that makes ownership usable, and the copyright-versus-patent trap that catches even sophisticated companies. It closes with the standing, co-ownership, and chain-of-title failures that surface downstream, a life-cycle drafting cadence, and the inventorship questions that generative AI now injects into the analysis. The recurring lesson: clean title is cheapest to secure on the first day of employment and most expensive to reconstruct when a deal or a lawsuit already depends on it.