At present spherical tokamaks are assumed to be prospective candidates for construction of thermonuclear reactors. These machines combine the advantages of spheromaks (compactness) and of tokamaks (improved plasma confinement). Such a combination allows achievement of higher plasma parameters in the presence of relative compactness and low cost of the main machine. Spherical tokamaks are also used for testing power stressed elements of the first wall and divertor under loads approaching those in experimental thermonuclear power reactors.