Nozzle shapes include two-dimensional and three-dimensional. If a wall is present near the jet, the jet will attach to it or impinge on it. Attached jets are used, for example, to control the boundary layer on a wall, while impinging jets are used to cool high-temperature walls. Depending on the shape of the wall or flow path, the jet may oscillate, which is useful, for example, as a fluid oscillator. Furthermore, when a jet is ejected at high speed, it exhibits the behavior of a compressible fluid, which is useful in jet engines and rocket engines. Volcanic eruptions and volcanic plumes are examples of jet phenomena in nature. Although only some examples are given here, jet phenomena have so many practical applications, and it is extremely important to clarify their basic flow and heat transfer characteristics. This chapter presents a basic fluid dynamics treatment of jet flow phenomena and discusses some application examples.