Group communication to and from sets of sensor nodes is an important paradigm in wireless sensor networks (WSNs). Securing this group communication is a difficult challenge given the energy-efficiency constraints posed by WSNs. In this paper, we introduce the protocol SLIMCAST, i.e. Secure Level key Infrastructure for MultiCAST and group communication, which uses level keys to provide an infrastructure that dramatically lowers the cost of nodes joining and leaving sensor groups. This level key infrastructure is shown to achieve energyefficient key updates that are localized for group multicast 1→N communication, and can be further leveraged to achieve secure group aggregation N→1 communication. Simulation results comparing the performance of SLIMCAST to traditional secure group communication protocols are presented to demonstrate SLIMCASTs energy efficiency and flexibility.