In a CDMA-based cellular network, all radio cells share the same frequency bands, and users can transmit simultaneously. Transmissions from one user causes interference to other users. The more users are in the system and the higher power they transmit, the more interference they generate to one another. A CDMAbased system is typically interference-limited. The management of transmission power and mutual interference is directly related to system capacity and qualityof- service (QoS) to the users. In this chapter, we first briefly review the motivations of power control in cellular CDMA networks, then study the power allocation problem in a single-cell CDMA network. Based on the analysis, different aspects that affect the transmission power and system capacity are investigated. We then study the power allocation problem in a multi-cell CDMA network. The relationship between transmission power, network capacity, and QoS to the users is analyzed.