The book is devoted to the results of investigations of a broad and widespread category of incompressible fluid motions in the form of laminar and turbulent jets. The development of computational methods applicable to an important type of jet streams and based on a consistent and systematic study of jet flows with a theoretical approach is the aim of the monograph. There are four parts to the book, including a foreword and an introduction. The first part deals with the solutions of jet problems based on the exact Navier-Stokes equations for incompressible fluids and, in particular, with the Landau investigation of the propagation of a submerged axially symmetric viscous fluid jet issuing from a thin tube. The second part contains a detailed analysis of laminar jet streams of an incompressible fluid by methods of boundary layer theory. In addition to free jets flowing into a stationary medium or into homogeneous wake flows, semibounded jets (wall jets) are considered. Turbulent jets of liquids and gases are the subjects of the third part, in which self-similar solutions for free and wall jet sources are investigated. It also contains detailed experimental data obtained under the guidance of one of the authors in thermophysical laboratories in Alma Ata for comparison with the theoretical results. The fourth part deals with certain theoretical and experimental problems of jet streams which may be regarded as complementary to the main topics treated in this book. Among them are complex turbulent jet streams, patterns of diffusion flames, and jets in magnetohydrodynamics. (Author).