Bacteria swim by rotating semirigid, left-handed helical flagellar filaments; counterclockwise (CCW) rotation produces straight swims, known as "runs," and clockwise (CW) rotation generates abrupt changes in direction, known as "tumbles." As a cell moves through its environment, alternately running and tumbling, it detects spatial gradients of attractants and repellents by making temporal comparisons of their concentration. These chemicals bind to receptors in the cell envelope to modulate the activity of the chemotactic signal transducers, proteins that span the cytoplasmic membrane. Signals generated by the transducers control the motion of the flagella to promote migration up attractant gradients and down repellent gradients. Chemotactic adaptation, accomplished by methylation-demethylation of the transducers, cancels out these signals. Adaptation is an essential component of the "memory" that allows bacteria to use a temporal mechanism to detect spatial gradients. Both signaling and adaptation are mediated by changes in the level of phosphorylation of several cytoplasmic chemotaxis (Che) proteins. The activity of the transducers regulates the rate of autophosphorylation of the CheA protein, which then passes the phosphate on to other proteins. In particular, phosphorylated CheY protein controls the frequency of tumbling because it promotes CW flagellar rotation, and the CheB esterase modulates adaptation because its nonphosphorylated form removes methyl groups from the transducers much more slowly than its phosphorylated form.