Two proposed IEEE standards for real-time operating systems support, POSIX.4 and POSIX.4a, are proceeding towards IEEE approval and will eventually become international standards. The authors provide a brief overview of the facilities of POSIX.4 and POSIX.4a. They concentrate on a few of the critical features that POSIX.4 and POSIX.4a provide and describe the POSIX.4 scheduling interface. The POSIX.4a support for multiple threads of control is also described. The features found in POSIX.4 and POSIX.4a for synchronization of multiple threads, are discussed, and the POSIX.4 interprocess communication facility is presented. The performance numbers are given to allow comparisons of the facilities of traditional Unix systems, the facilities of a representative hard real-time system (LynxOS), and the facilities of POSIX.4 and POSIX.4a.>