UAVs, commonly referred to as drones, have been a topic of focused research over the past few years, due to their autonomy, flexibility, and a wide range of uses. Specifically, UAVs have been recognized as key tools for numerous applications, including military operations, surveillance and monitoring, telecommunications, medical supply delivery, and rescue missions. These recent advances in drone technology make it possible to widely deploy UAVs, such as drones, small aircraft, balloons, and airships for wireless communication purposes. UAVs can be integrated into wireless communication networks, either as flying (BSs) or as mobile relays. While UAVs offer significant advantages in both roles, this section focuses specifically on their application as relays.