Active networks research proposed extending Internet-class systems by scripting in-network packet processing. We apply that idea to sensor networks, arguing that retasking a deployment is effectively active sensor networking. As a sensor network deployment is designed for a particular application domain, we propose using application specific virtual machines (ASVMs) to enable safe execution, concise code, and efficient operation. We present Mate', an architecture for building ASVMs for active sensor networks. We show how Mate'-based sensor networking is flexible enough to support existing proposals for higher-level sensor network programming, such as SQL queries, with improved efficiency and safety over hand-built implementations of those proposals.