Swarming involves differentiation of vegetative cells into hyperflagellated swarm cells that undergo rapid and coordinated population migration across solid surfaces. Cell density, surface contact, and physiological signals all provide critical stimuli, and close cell alignment and the production of secreted migration factors facilitate mass translocation. Flagella biogenesis is central to swarming, and the flhDC flagellar master operon is the focal point of a regulatory network governing differentiation and migration.