Water-swellable elastomers (WSE) constitute a class of rubbery materials that have been widely studied both in academia and industry during the last 25 years. Market pull is the major driver for the exploration of these materials. The need of WSE in several sealing applications has driven the attention of many academic researchers toward the possibility to provide a rubber with water-swelling characteristics. As commercial rubbers are hydrophobic materials, making them swell in water presents an interesting and difficult challenge. This paper reviews the scientific and patent literature on the fundamental aspects of WSE: the various synthetic approaches, the properties of the corresponding polymers (not only the swelling performance but also the mechanical behavior), and some of their applications. Particular attention is paid to the chemical structure/performance relationships of WSE. Finally, the authors speculate on a great future for WSE that can be rationally designed for improved and/or new applications.