• Monitors are a higher level (compared to semaphores) synchronization primitive proposed independently by C.A.R. Hoare(1974) and Brinch Hansen (1975)

Monitor is a collection of procedures, variables, and data structures grouped together (i.e., an object), which  provides automatically mutual exclusion (only one process can be active in a monitor at any instant of time) and condition synchronization using condition variables

• Mutual exclusion on monitor entries is usually implemented using a binary semaphore and is provided implicitly and automatically, thus not a programmer has to implement it
• Because the compiler, not the programmer, is arranging for the mutual exclusion, it is less likely that it will make errors with the implementation of mutual exclusion
• For synchronization of processes using monitors, condition variables are used
• All together, monitors provide implicit mutual exclusion (with minimal mutually excluding sets of size two) and explicit condition synchronization by condition variables signaling
• Monitors are available in several programming languages, including Java, Concurrent Pascal (Brinch Hansen), Modula-1/2/3 (Wirth), Pascal Plus,Mesa(Xerox Parc), CSP/k (based on PL-1), Concurrent Euclid, Turing Plus (all 3 designed by Holt from U. of Toronto), Emerald, Ada 95 (with protected types) and Solve (UCL, ESPRIT SPAN).
• Locks and condition variables are also a part of the POSIX Pthreads standard library, which allows to simulate monitors.