  The deployment of Aerial Base Stations (ABSs) mounted on board Unmanned
Aerial Vehicles (UAVs) is emerging as a promising technology to provide
connectivity in areas where terrestrial infrastructure is insufficient or
absent. This may occur for example in remote areas, large events, emergency
situations, or areas affected by a natural disaster such as a wildfire or a
tsunami. To successfully materialize this goal, it is required that ABSs are
placed at locations in 3D space that ensure a high quality of service (QoS) to
the ground terminals. This paper provides a tutorial introduction to this ABS
placement problem where the fundamental challenges and trade-offs are first
investigated by means of a toy application example. Next, the different
approaches in the literature to address the aforementioned challenges in both
2D or 3D space will be introduced and a discussion on adaptive placement will
be provided. The paper is concluded by discussing future research directions.
