A hands-on, production-minded guide to migrating real systems to post-quantum cryptography, written from the practitioner’s chair rather than the lecture hall. It follows the NIST standards (FIPS 203/204/205) and the deployment reality behind them: what the algorithms cost, how to move TLS and PKI, what auditors ask, and how a live zero-knowledge platform actually shipped a hybrid post-quantum migration. The production case studies, including the Envelope v3 post-quantum migration, are drawn from Etergis, a shipped, verified codebase, not a simulation built for the book.