Bacterial chemotaxis, which has been extensively studied for three decades, is the most prominent model system for signal transduction in bacteria. Chemotaxis is achieved by regulating the direction of flagellar rotation. The regulation is carried out by the chemotaxis protein, CheY. This protein is activated by a stimulus-dependent phosphorylation mediated by an autophosphorylatable kinase (CheA) whose activity is controlled by chemoreceptors. Upon phosphorylation, CheY dissociates from its kinase, binds to the switch at the base of the flagellar motor, and changes the motor rotation from the default direction (counter-clockwise) to clockwise. Phosphorylation may also be involved in terminating the response. Phosphorylated CheY binds to the phosphatase CheZ and modulates its oligomeric state and thereby its dephosphorylating activity. Thus CheY phosphorylation appears to be involved in controlling both the excitation and adaptation mechanisms of bacterial chemotaxis. Additional control sites might be involved in bacterial chemotaxis, e.g. lateral control at the receptor level, control at the motor level, or control by metabolites that link central metabolism with chemotaxis.