Occasionally, someone will toss out in discussion “That’s an ABI breaking change”. What does that mean? Is that a problem? In this talk, I’ll walk though what an ABI is, talk (a lot) about the "one definition rule", and give examples, and the a definition of an "ABI break”. I’ll talk about history (libstdc++ and string, and others), and talk about some things that the C++ committee is considering.