Swarming is a form of active surface motility that is widespread among flagellated, Gram-negative bacteria. In the laboratory, growth of the bacteria on certain agar surfaces leads to induction of the differentiated swarmer-cell state. Swarmer cells are generally long and multinucleate, always hyperflagellated, and can move rapidly over the agar surface in a coordinated manner. Some swarm colonies exude large amounts of 'slime', which could be essential for promoting intimate cell-cell contacts during swarming. There is evidence that the differentiated swarmer-cell stage facilitates pathogenic associations with host tissue. Almost nothing is known about the molecular signalling mechanism of surface sensing. Increased viscosity appears to be sensed by several bacteria, but other environmental cues, specific to each bacterium, are also important. In organisms in which swarming motility has been studied in some detail, the chemotaxis system has been shown to play an important role. The recent discovery of swarming motility in two genetically well-characterized organisms--Escherichia coli and Salmonella typhimurium--should lead to rapid progress in understanding this process.