Although the interest in Wireless Sensor Network research is increasing, and new concepts and applications are being demonstrated, several fundamental issues remain unsolved. While many of these issues do not require to be solved for proof-of-concept designs, they are important issues to be addressed when referring to the long-term operation of these systems. One of these issues is the system''s lifetime, which relates to the lifetime of the nodes, upon which the system is composed. This book focuses on node lifetime extension based on energy management. While some constraints and results might hold true from a more general perspective, the main application target involves environmental measurement systems based on Wireless Sensor Networks. We show how precise task scheduling in these systems, due to node synchronization, combined with methods such as duty cycling and power domains, can optimize the overall energy use. Furthermore, we address power generation from ambient energy sources, with a focus on solar energy harvesting systems at locations with limited insolation.