Recently, energy-scavenging studies have become an important research area not only for the utilization of ambient energy sources but also for their high potential to replace batteries, especially for wireless body area networks (WBANs). Conventionally, batteries are used to provide energy to WBANs. The amount of energy provided by batteries limits on-board features and communication range of WBANs. Also, battery recharging or replacement is significantly impractical. Therefore, different energy-scavenging interface circuits with different efficiencies have been proposed to overcome these limitations. This chapter focuses on energy scavengers and their potential utilization for low-power systems. Particularly, energy scavenging-interface circuits for WBANs are investigated in this survey chapter.