Flagella are complex surface organelles that allow bacteria to move towards favourable environments and that contribute to the virulence of pathogenic bacteria through adhesion and biofilm formation on host surfaces. There are a few bacteria that possess functional dual flagella systems, such as Vibrio parahaemolyticus, some mesophilic Aeromonas spp., Rhodospirillum centenum and Azospirillum brasilense. These bacteria are able to express both a constitutive polar flagellum required for swimming motility and a separate lateral flagella system that is induced in viscous media or on surfaces and is essential for swarming motility. As flagella synthesis and motility have a high metabolic cost for the bacterium, the expression of the inducible lateral flagella system is highly regulated by a number of environmental factors and regulators.