Eukaryotic cells predominantly use serine, threonine, and tyrosine phosphorylation in various intracellular signal transduction pathways. In contrast, prokaryotic organisms employ numerous "two-component" systems, in which signaling is achieved by transferring a phosphoryl group from phosphohistidine in the "sensor kinase" component to aspartate in the "response regulator" component. In the last several years, genetic screens and genome projects have identified sensor kinases and response regulators in lower eukaryotes and plants, revealing that eukaryotic organisms also make use of His-Asp phosphotransfer in a limited number of signaling pathways. Extensive studies in yeasts have demonstrated that a variation of the two-component system, a multistep "phosphorelay," is the prevailing mechanism among distantly related yeast species. In the budding yeast Saccharomyces cerevisiae, a His-Asp-His-Asp phosphorelay transmits osmotic stress signals to a mitogen-activated protein kinase (MAPK) cascade to induce adaptive responses. A phosphorelay in the fission yeast Schizosaccharomyces pombe, analogous to the S. cerevisiae phosphorelay, is responsible for MAPK activation in response to peroxide stress. Mammalian cells do not have any two-component or phosphorelay systems, although protein histidine kinases unrelated to the sensor kinase may be involved in cellular signaling. Because some phosphorelay proteins are essential for virulence of microbial pathogens, including the yeast fungus Candida albicans, novel antibiotics targeted to phosphorelays may be effective against eukaryotic pathogens without causing host cell damage.