Thermodiffusion is a coupled mass and heat transfer phenomenon in which the constituents in a mixture develop a preferential separation direction in the presence of a thermal gradient. It occurs in numerous areas in nature and is also employed as an underlying principle in the separation processes in engineering fields. In space platforms like the International Space Station, this phenomenon has shown some promise as a mode of transport of fluids. This chapter will introduce the phenomenon of thermodiffusion as it is studied in the various mixtures in different disciplines. Application and significance of thermodiffusion are also presented to outline the motivation behind the thermodiffusion studies. We also briefly highlight the challenges in some of the studies that are conducted in outer space. The chapter concludes with an outline of the rest of the book.