A copyright in software exists the moment code is written, but the ability to enforce that copyright—to sue, and to collect the statutory damages and attorney's fees that make suing worthwhile—turns on registering it with the U.S. Copyright Office. This guide explains what copyright in code actually reaches (and the wide band of functional material it leaves free), drawing on Section 102(b), the abstraction-filtration-comparison test, Google v. Oracle, and the reverse-engineering cases. It walks the eCO application, the source-code deposit rules, and the trade-secret redaction options that let developers register without exposing their crown jewels. It covers ownership and work made for hire, preregistration and derivative versions, screen displays and GUIs, video games, documentation, HTML, object code and the Rule of Doubt, special relief when the code is lost, and how a certificate is put to work proving infringement. Written for developers, founders, in-house counsel, judges, and curious non-lawyers alike, with fresh worked examples throughout.