High hydrostatic pressure has the potential to produce high quality foods that are microbiologically safe and with an extended shelf-life. Micro- organisms vary in their response to high pressure. Bacterial spores are the most resistant group and they cannot be significantly inactivated by pressure alone. Combination treatments using high pressure and heat have been proposed as a method of producing shelf-stable low acid foods. Viruses are less resistant than bacterial spores and their infectivity can be abolished without destroying their ability to elicit antibodies, leading to the possibility of vaccine production. Yeasts, moulds and vegetative bacteria vary in their response to pressure, depending on factors such as species, strain, processing temperature and substrate, and these are reviewed in the paper. A knowledge of how these factors interact is necessary in order to select the optimum processing conditions for foods. A number of pressure-treated foods are already commercially available and these are discussed in the paper.