The problems of a free jet, a trace of a poorly streamlined body, a jet constrained by hard walls, etc. are considered. A systematic analysis of numerous experimental data on the velocity, temperature and impurity concentration profiles, as well as the outline of the turbulent mixing zone is given. The theory of turbulent gas jets, including highly heated and supersonic ones, is presented. Several problems have been solved about the spread of a jet in a limited and semi-limited space. It also contains various applications of jet theory, many of which are first developed or redesigned.