Unmanned aerial vehicles (UAVs) have found many important applications in communications systems. In these systems, they can serve as either aerial base stations or mobile relays to improve the quality of services. In this seminar, we study the use of multiple UAVs in relaying. Considering two typical uses of multiple UAVs as relays that form either a single multi-hop link or multiple dual-hop links, we first optimize the placement of the UAVs by maximizing the end-to-end signal-to-noise ratio for two commonly used relaying protocols. Based on the optimum placement, the two relaying setups are then compared in terms of outage and bit error rate. Numerical results show that decode-and-forward UAVs provide better performances than amplify-and-forward UAVs. The investigation also reveals the effects of important system parameters on the optimum UAV positions and relaying performances to give us useful guidelines on how to place the UAVs.