The POSIX standard promotes portability of applications across different operating system platforms. This is especially important for applications designed for longevity, where the hardware and software infrastructure may change during the application’s life cycle. However in real-time systems, where predictability and low overhead are important, portability is often sacrificed. In this paper we will discuss the use of POSIX in real-time systems, including the POSIX real-time and thread extensions. We will first discuss what POSIX covers, and the differences that still exist between operating system implementations. We will then look at the performance of various POSIX mechanisms, using as case studies two general purpose operating systems and two real-time operating systems.