TheReconfigurable Intelligent Surface (RIS) use of unmanned aerial vehicles (UAVs) as flying base station in wireless communication has attracted a lot of interest in research and industry. UAV as a base station has more coverage as compared to the ground base station and has a direct or line-of-sight (LOS) communication link with most of the ground users. In such scenarios, propagation losses are minimum and the users are served with the least possible amount of transmission power. However, it may sometimes occur that there is no LOS communication link, i.e., an obstacle comes between the UAV and the ground user causing an invisibility. This results in more propagation losses and high transmission power is needed for communication between UAV and ground users. Fortunately, in no LOS link cases, the transmission power can be minimized by using reconfigurable intelligent surfaces (RIS)Reconfigurable Intelligent Surface (RIS). These surfaces have the ability to reconfigure the reflection of the intended signal coming from the UAV and send it to the desired users by adjusting its direction and phase. The direct mode of communication (i.e., from the UAV to ground users) and the reflecting mode of communication (i.e., from UAV to RIS and RISReconfigurable Intelligent Surface (RIS) to ground users) consume different amounts of power, and one between the two should be selected intelligently to capitalize on the use of RISReconfigurable Intelligent Surface (RIS) joint with UAV. In this chapter, we consider a wireless communication model with an RISReconfigurable Intelligent Surface (RIS)-assisted UAV base station to serve the multiple ground users. We consider the problem of communication mode selection and power allocation with objective of maximizing the bits per joule to maximize the system energy efficiency. For this purpose, we formulate a mathematical problem and find a solution to that problem. Then, with the help of numerical simulations, we evaluate the energy efficiency performance of our proposed solution.