In general, protein phosphorylation is one of the most widely used mechanisms for regulating biological processes, including intracellular signal transduction. In eukaryotes, the cascades of protein phosphorylation and dephosphorylation events involving a number of protein tyrosine or serine/threonine kinases have been well studied. In contrast, recent intensive studies revealed that bacteria have devised a quite different phosphotransfer signaling mechanism for eliciting a variety of adaptive responses to their environment. Such a bacterial signal transduction mechanism was originally referred to as a "two-component regulatory system." The mode of molecular communication between a "sensor kinase" and its cognate phospho-accepting "response regulator" is principally based on histidine-to-aspartate (His-Asp) phosphotransfer. In Escherichia coli, for example, at least 30 different sensor-regulator pairs operate in a wide variety of adaptive responses. This particular signal transduction mechanism was once thought to be restricted to prokaryotes. However, many instances have recently been uncovered in diverse eukaryotic species. Furthermore, recent studies suggested that the molecular mechanism underlying the bacterial signal transduction is not simple as, and, in fact, is more sophisticated than thought previously. The new concept should be referred to as the "multi-step His-Asp phosphotransfer signaling mechanism."