Hydrogels are crosslinked macromolecules in which at least part of the network consists of hydrophilic groups. Hydrogels are widespread in nature; for example, plant and animal tissues consist mainly of hydrogels. They are becoming increasingly important in chemical engineering, e. g, as ion excahngers, in gel chromatography, and for dewatering. Hydrogels swell or shrink in aqueous solutions. Such swelling equilibria are the basis for most of their applications. This article surveys the applications and also the swelling behaviour of hydrogels. It covers both the termodynamic principles and methods for correlation of experimentally determined swelling equilibria for nonionic and ionic hydrogels. The swelling of a hydrogel results from the interplay of intermolecular interactions in aqueous, partly ionic polymer solutions on the one hand and elastic properties of polymeric networks on the other. It ist therefore hardly surprising that it is presently impossible to quantitatively predict phase equilibria and the swelling behaviour of such hydrogels from a few basic data. However, it is possible to describe the swelling of hydrogels with straightforward approaches reported in the literature if a number of the parameters of these approaches are adapted to experimentally determined swelling data.