Most applications of wireless sensor networks (WSNs) require long term (several months or even years of) unattended operation from their networks. But, significant challenges still remain in achieving long-term unattended operation from large-scale wireless sensor networks. Critical among those are the problems of power management. An important distinction between wireless sensor networks and most existing systems is the tremendous gap between the energy available to a sensor node and that required for its long-term unattended operation. A typical sensor node such as ones from the Mica family can last 3{5 days on a pair of AA batteries in its fully active mode. In real life applications, though, we would like a sensor network (comprised of these motes) to last at least a few months. The story is not very different with other platforms. The question then is: How can we fill this huge energy gap? The approaches and techniques applied to fill this energy gap are collectively referred to as Power Management in wireless sensor networks.