Some salient characteristics of microbial osmoregulation are reviewed, with specific examples drawn from eukaryotes. As well as the need for an osmoregulatory solute to be 'compatible' with cellular processes under all conditions, the importance of the physiological method of regulating the content of the solute as a factor determining xerotolerance is emphasized. The significance of turgor/volume homeostasis is discussed and examples are cited in which, during exponential growth, there is apparently no homeostatic control of the cellular content of the major osmoregulatory solute. Some implications of this for the overall mechanism of osmoregulation are considered. A recent experiment is described which raises questions about the timing of an osmoregulatory 'signal' in Saccharomyces cerevisiae. Other experiments are summarized which distinguish between osmoregulatory and compatible solutes in yeast. These experiments implicate trehalose as a non-osmoregulatory compatible solute in certain circumstances. © 1986.