The hydrostatic pressure is one of the important state variables in phase equilibrium, and its generation techniques have been so much developed that it is now possible to study in detail the effect of pressure on phase transformations. In this overview, the effects of pressure on phase transformations, especially on martensitic transformations, are described in detail, centering around our recent systematic work, since there are still a small number of work in this research field. The effect of hydrostatic pressure on martensitic transformation is fundamentally brought about through an interaction of the hydrostatic pressure with the volume change, expansion or contraction, accompanying the transformations. The volume change is affected by the invar effect in ferrous invar alloys. Therefore, the hydrostatic pressure effect on martensitic transformations may be varied with the type of the martensitic transformations, thermoelastic or non-thermoelastic, athermal or isothermal, and it also depends on the kind of alloys, invar or non-invar. From this point of view, the hydrostatic pressure effects have been examined for those various types of martensitic transformations in various kinds of alloys, which include Fe–Ni, Fe–Ni–C, Cu–Al–Ni, disordered and ordered Fe–Pt, aged Ti–Ni, ausaged Fe–Ni–Co–Ti and Fe–Ni–Mn alloys. It is clearly shown that the martensitic transformation start temperature, M_s, is decreased or increased with increasing hydrostatic pressure, depending on whether the volume change is expansion or contraction. These changes in M_s are in good agreement with theoretical calculations in which hydrostatic pressure dependences of volume change and invar effect are both taken into consideration, using an equation derived by modifying that of Patel and Cohen. Despite those changes in M_s, martensite morphology is not varied with the hydrostatic pressure, irrespective of the type of martensitic transformations and the kind of alloys.