Most sensor network applications are dominated by the acquisition of sensor values. Due to energy limitations and high energy costs of communication, in-network processing has been proposed as a means to reduce data transfers. As application demands may change over time and nodes run low on energy, get overloaded, or simply face debasing communication capabilities, runtime adaptation is required. In either case, it is useful to be able to migrate computations between neighboring nodes without losing runtime state that might be costly or even impossible to recompute. We propose stateful mobile modules as a basic infrastructure building block to improve adaptiveness and robustness of in-network processing applications. Stateful mobile modules are binary modules linked on the node itself. Even more importantly, they can be transparently migrated from one node to another, thereby keeping statically as well as dynamically allocated memory. This is achieved by an optimized binary format, a memory-efficient linking process and an advanced programming support.