An unmanned aerial vehicle (UAV) - aka drone, unmanned aircraft system or remotely piloted aircraft system - is an aircraft without a human pilot on board. Its flight can be controlled autonomously by computers in the vehicle or by remote control. UAVs can uniquely penetrate areas, which may be too dangerous or too difficult to reach for piloted craft. The UAV cyber-physical system comprises all the subsystems and interfaces for processing and communication functions performed by the embedded electronic system (avionics) and the ground control station. To accomplish the desired real-time autonomy, the avionics is highly tied with aerodynamics sensing and actuation. An entirely autonomous UAV can (i) obtain evidence about the environment, (ii) work for an extended period of time without human interference, (iii) move either all or part of itself all over its operating location devoid of human help and (iv) stay away from risky situations for people and their assets. This chapter intends to introduce the material addressed in further chapters of this book. The next sections go through some concepts that are recurrent in the book.