The histidyl-aspartyl phosphorelay, formerly described as the two-component system, is the predominant mode of signal transduction in bacteria. Adaptation to environmental changes occurs through a sensor histidine protein kinase and a response regulator. The histidine protein kinase is usually a transmembrane receptor and the response regulator is a cytoplasmic protein. Together the histidyl-aspartyl phosphorelay proteins mediate reversible phosphorylation events that control downstream effectors. Following autophosphorylation at a conserved histidine residue, the histidine kinase serves as a phospho-donor for the response regulator. Once phosphorylated, the response regulator mediates changes in gene expression or cellular locomotion. The EnvZ-OmpR phosphorelay system in Escherichia coli, which monitors external osmolarity and responds by differentially modulating the expression of the OmpF and OmpC major outer membrane porins, will be described as a model system. While histidine kinases were thought to be present only in prokaryotes, they have recently been identified in eukaryotic systems. Here, we review the unique and conserved features of this growing family of signal transducers.