Though mobility is rarely considered in traditional wireless sensor networks (WSNs), actively exploiting mobility to improve the performance of WSNs has been increasingly recognized as an important aspect of designing WSNs. This chapter focuses on exploiting mobility to improve the network lifetime of a WSN. We present a general optimization framework that is able to capture several aspects of maximizing network lifetime (MNL) involving mobile entities. Based on this framework, we conduct an in-depth analysis on each of these aspects and also describe algorithms that can be used to solve the resulting optimization problems. We also present certain numerical results where engineering insights can be acquired.