Wireless sensor networks (WSNs) consist of a large number of sensor nodes, and are used for various applications such as building monitoring, environment control, wild-life habitat monitoring. forest fire detection, industry automation, military, security, and healt-care, each sensor node needs an operating system (OS) that can control the hardware, provide hardware abstraction to application software, and fill in the gap between application and the underlying hardware. In this paper, researches present os for WSNs