Book excerpt: Wireless sensor networks are expected to find wide applicability and increasing deployment in coming years, as they enable reliable monitoring and analysis of the environment. Users of such a system will expect to get a real time warning when time-critical situations occur in the network and also to be able to retrieve any required information by issuing queries to the network. In this thesis, we propose a formal classification of sensor networks, based on their mode of functioning, as proactive and reactive networks. In proactive networks data is collected at pre-specified, fixed intervals while reactive strategy requires the nodes to respond immediately ton changes in the relevant parameters of interest. We are able to combine the best features of proactive and reactive networks while minimizing their limitations to create a new type of network called a Hybrid network which not only reacts to time-critical situations, but also gives an overall picture of the network at periodic intervals in a very energy efficient manner. We introduce two new energy efficient protocols, TEEN (Threshold-sensitive Energy Efficient sensor Network protocol) for reactive networks and APTEEN (Adaptive Periodic Threshold-sensitive Energy Efficient sensor Network protocol) for hybrid networks. We have also proposed a third protocol for routing queries in hybrid networks. This protocol provides the user, flexibility to request either past, present or future data from the network in the form of historical, one-time and persistent queries respectively. We have also analytically determined the delay incurred in handling the various types of queries. To our knowledge, such an analytical modeling has been done for the first time for sensor network queries. These three protocols offer versatility to the users while consuming energy very efficiently by minimizing non-critical data transmissions. We evaluated the performance of these protocols for a simple temperature sensing application with a Poisson arrival rate for queries. In terms of energy efficiency, these protocols have been observed to outperform existing conventional warehousing sensor network protocols.