Astronauts are people trained by the human space program to command, drive or act as crew members of spacecraft. Although usually reserved exclusively for professional space travelers, the term is sometimes applied to anyone who enters space, including scientists, politicians, journalists and tourists.

From the 1950s to 2002, astronauts were fully sponsored and trained by the government (by military or civil space agencies). With the suborbital flight of SpaceShipOne in 2004, a new category of astronauts was created: commercial astronauts.