Deep-sea environment have selected for the evolution of unusual bacterial extremophiles which are adapted to life at high pressures. This review briefly characterises barophilic bacteria; compares and contrasts high pressure effects on cellular and biochemical processes in both barosensitive and barophilic bacteria; and presents molecular and genetic approaches which have been used to examine the basis of high pressure sensitivity in terrestrial barosensitive bacteria, or to dissect barophilic processes in deep-sea bacteria.