The interest in jets expanding into a vacuum or a low-density background is raised by the appearance of new technological methods and growing requirements of high technology development. The use of free jets for vacuum technology, including space applications, has a number of aspects. One of the important achievements of the previous investigations is an understanding of jet structures in a variety of gas dynamic conditions. The accumulated knowledge on radial expansion into a vacuum, on the expansion of jets behind axially symmetric and flat sonic, and supersonic nozzles into a vacuum and background from the literature can be used for approximate evaluations. At present. more detailed data can be obtained as the result of computational modeling by continuum and kinetic approaches. Modern gas dynamics has at its disposal effective theoretical methods that open wide possibilities for vacuum technology development. The present article reflects the accumulated knowledge on the structure of jets, calculation of jet parameters, and modern trends in the analysis of low density jets.