This chapter focuses on the performance limits and metrics for wireless networks that integrate UAV base stations. In particular, we first have a brief overview on performance analysis techniques such as stochastic geometry. Then, we introduce several detailed case studies to analyze the performance limits of wireless communications with UAVs, while uncovering important design tradeoffs and exposing the impact of various unique UAV features such as altitude, mobility, line-of-sight communications, and elevation angle, on the various metrics.