SMEPP Light is a middleware for Wireless Sensor Networks (WSNs) based on mote-class sensors. It is derived from the specification developed under the framework of the SMEPP project, to deal with the hardware and software constraints of WSNs. SMEPP Light features group management, grouplevel security policies, mechanisms for query injection and data collection based on a subscribe/event mechanism, and adaptable energy efficiency mechanisms. In this paper we present the SMEPP Light specification, its architecture and its main protocols for communication, security, and energy efficiency. General Terms Wireless Sensor Networks, Middleware, Embedded Systems, Energy Efficiency