Embedded systems are found in all market segments: consumer commercials, PC peripherals, telecommunications, automotive and in the fields of aerospace and nuclear research. It is a very demanding technology where products must meet special requirements such as cost effectiveness, low power, small footprint and a high level of system integration. The embedded systems mostly work in a real time environment where deadlines should not be missed at any instance. The platform to execute this real time application software is known as a Real Time Operating System (RTOS). RTOS consists of a priority based, preemptive, multitasking scheduler to manage the interrupts. Here the requirements of an OS to be a RTOS are discussed and the facilities available for a widely used RTOS VxWorks are also overlooked. Thus we can say that the basic requirements of an OS to be a RTOS are its fast intertask synchronization and communication capabilities, priority based preemptive interrupt handling capability etc. Also comparing to a general OS RTOS only requires a small footprint. It is a modular OS.