The knowledge on gas expansion into vacuum or a low density background was of a paramount importance for the development of vacuum technologies throughout their history. The variety of gas sources, of flow parameters in sources and parameters of background gas state sets the problem of jet flows as vast for the analysis. But the classification of flows is possible. The first step is division of gas expansion into vacuum and in a background. The expansion into vacuum can be considered separately for different types of sources (orifices, capillaries, flat slots, Laval nozzles) under different channel Knudsen numbers. The discharge coefficients, the plume of molecular flow, the structure of the free jet, relaxation processes in the jet, the gas mixture issue represent the main interest. By the expansion of gases into a background from some source the structure of jets in the regimes from free molecular to turbulent flow is defined by the source and state of the background gas. The available results of investigations allow to determine the location of different zones and character of flow, including the evaluation of relaxation of the internal energy and condensation effects. New problems of vacuum technologies are in need of determination of the flow parameter distribution with high accuracy. New computational methods can be used for this purpose. The direct simulation Monte Carlo (DSMC) method became a powerful instrument for study of jet flows. It has depreciated attempts to extend the continuum approach on cases of flows with effects of rarefaction. The hybrid DSMC / fluid methods raise the role of computational simulation, taking into account difficulties of experiments. New achievements in theory can provide not only optimization of technology but extraction of physical knowledge by industrial testing as well.