More than ever, commercial, government, and military organizations are demanding that developers use POSIX interfaces. The question is, how much of the POSIX.1 standard must your project support? POSIX.1 is so large, and has so many optional components, that few applications need everything it offers. This paper explores how to: identify which POSIX APIs your system requires; determine whether your OS can support those APIs; achieve portability without sacrificing performance; and weigh the relative merits of POSIX conformance, POSIX compliance, and POSIX certification.