The document discusses future trends and research challenges in wireless sensor networks (WSNs). It begins by providing background on WSNs and their applications. Recent advances that have enabled new applications are then described, including in areas like localization, military uses, industrial monitoring, traffic management, structural health, and agriculture. Future trends discussed include cognitive sensing, spectrum management, underwater acoustic sensor systems, and coordination in heterogeneous networks. The document concludes by stating time critical applications are an emerging area for WSNs.