IntroductionMotility is one of the most impressive features of microbial life and requires a large percent of cellular energy. Its roles include increased efficiency of nutrient acquisition, evasion of toxic substances, ability to translocate to preferred hosts and access to optimal colonization sites within them, as well as dispersal in the environment. Movement in aqueous environments by swimming or along surfaces by using different modes of translocation has been classified into several distinct types [1]. Pseudomonas aeruginosa exhibits three types of motility -flagellum-mediated swimming, flagellum and type IVpilus-mediated swarming [2,3], and type IVpilus-mediated twitching [4][5][6]. Swarming motility can be distinguished from swimming motility in that swarming is required to move across a hydrated, viscous semisolid surface, while swimming allows movement through a relatively low-viscosity liquid environment. The direction of movement is biased by chemotactic responses to chemical stimuli [7][8][9]. This chapter focuses on the two motility organelles of P. aeruginosa, i.e. the flagellum and the pilus, their structure, regulation and role in pathogenesis. 4.2 Flagellum of P. aeruginosa P. aeruginosa has a single, unsheathed polar flagellum similar to that of Caulobacter crescentus and Vibrio parahaemolyticus, but unlike the polar, sheathed flagellum observed in Helicobacter and the peritrichous flagella seen in Escherichia coli and Salmonella enterica serovar Typhimurium. However, swarmer cells of P. aeruginosa appear to possess on average two polar flagella and no lateral flagella [2,10]. At the genetic level, the expression of flagellar genes in P. aeruginosa is tightly regulated, involving transcriptional activators and multiple s factors as in other enteric bacteria. Flagellar structural genes are usually conserved across the genomes Pseudomonas. Model Organism, Pathogen, Cell Factory. Edited by Bernd H.A. Rehm