The Unmanned Aerial Vehicle (UAV) or pilotless or uninhabited/uncrewed aircraft is a remotely piloted vehicle (RPV) or self-piloted aircraft that can carry payloads such as cameras, sensors, and communications equipment. All flight operations (including take-off and landing) are performed without on-board human pilot. In some reports of DOD, Unmanned Aerial System (UAS) is preferred. In media reports, the term “drone”, flying robot, or aerial robot are utilized. In this chapter, definitions; design process; UAV classifications; current UAVs; and challenges; will be covered. In addition, conceptual design, preliminary design and detail design of an UAV based on systems engineering approach are introduced. In each stage, application of this approach is described by presenting the design flow chart and practical steps of design.