Phosphotransfer-mediated signaling pathways allow cells to sense and respond to environmental stimuli. Autophosphorylating histidine protein kinases provide phosphoryl groups for response regulator proteins which, in turn, function as molecular switches that control diverse effector activities. Structural studies of proteins involved in two-component signaling systems have revealed a modular architecture with versatile conserved domains that are readily adapted to the specific needs of individual systems.