Gravity waves are considered to be important for dynamics and chemistry in the polar middle atmosphere. However, there are only a limited number of studies of gravity waves in the polar region mostly because of difficulty of their observation. One of the significant issues on gravity waves is their source, because wave parameters such as wavelengths and phase velocities which are essential to evaluate the gravity wave effects on the large-scale field are highly dependent on the source. This paper describes dominant sources of gravity waves in the polar regions, namely topography, the polar- night jet, and tropical convection, by reviewing results from recent studies
