Technological advancements in recent years have enabled the development of tiny, cheap, disposable, and self contained battery powered computers, known as sensor nodes or "motes", that can accept input from an attached sensor, process this input and transmit the results wirelessly to some interested device(s). When a number of these nodes work together, conceivably up to hundreds of thousands, a Wireless Sensor Network (WSN) is formed. Creating an application to run on a WSN is a not an easy task, with many issues, including power conservation, wireless communication, very limited RAM, etc., needing to be addressed. Over the last five years or so a number of operating systems have been developed to aid developers. For many applications of wireless sensor networks the required lifetime of the sensor nodes may be weeks, months or even years and battery recharging or replacement is unlikely to be feasible, especially in large scale deployments with thousands of widely dispersed nodes, or for nodes placed in hazardous environments. For this reason the power management features are a very important consideration when choosing an operating system for a particular application. We review the current state of operating systems for sensor nodes and compare their power consumption on a popular hardware platform while performing common tasks, e.g. transmitting data, processing data, etc. © 2007 IEEE.