This paper, presents a survey on current state of the art in Wireless Sensor Networks (WSNs) Operating Systems (OSs). WSN is composed of miniature senor and resource constraint devices. WSN is highly dynamic network because nodes die out due to severe environmental conditions and battery power depletion. Stated characteristics of WSN impose additional challenges on OS design for WSN. Therefore; OS design for WSN deviates from traditional OS design. The purpose of this survey is to point out strengths and weaknesses of contemporary OS for WSNs, keeping in view the requirements of emerging WSNs applications. State of the art, in operating systems for WSNs has been examined in terms of Architecture, Scheduling, Threading Model, Synchronization, Memory Management, and Communication Protocol support. The examination of these features is performed for both real time and non real time operating systems for WSNs. We believe that this survey will help the network designers and programmers to choose the right OS for their network and applications. Moreover, pros and cons of different operating systems will help the researchers to design more robust OSs for WSNs.