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For many pathogenic bacteria, flagellum-dependent motility and chemotaxis are crucial factors in the process of colonization of the host organism and establishment of a successful infection. Structural components of the flagellum as well as secretory and regulatory proteins involved in the synthesis of the flagellar apparatus and the control of chemotaxis have been analyzed in recent years, and knowledge regarding the mechanisms of Helicobacter pylori motility is rapidly increasing. The chapter summarizes the available experimental and genomic data on flagellar function and provides an overview of the current knowledge of this field. Transcription of s54-dependent genes depends on upstream enhancer-like sequences, which are bound by a class of transactivating proteins usually referred to as the NtrC (NR-1) family. Characterized loci include the fliI-fliQ operon coding for two flagellar export proteins and a stress-responsive operon containing the gene encoding the chemotaxis regulator CheY. Both of these operons have been shown to be regulated by a σ80-dependent transcription, and sequence analysis of the H. pylori genome suggests that the expression of the other flagellar biosynthetic, basal-body, and chemotaxis genes also is regulated by the same sigma factor. The regulation of chemotaxis has been extensively studied in the model organisms Escherichia coli and Salmonella enterica serovar Typhimurium. In these bacteria, sensing of chemoattractants or -repellents is mediated by methyl-accepting chemotaxis proteins (MCP), which possess a periplasmic ligand interaction domain and a large cytoplasmic signaling and adaptation domain.