High-pressure research has become increasingly important, and high-pressure techniques are an invaluable tool for synthesizing new materials and discovering novel physical phenomena that are not accessible at ambient pressure. This text deals with the problem of phase transitions in solids at high pressure. It covers phase equilibria at high pressures in various materials, the deformation impact on phase transformations at high pressures, high-temperature and low-temperature transformation kinetics and hysteresis at high pressures, and the requirements for obtaining and maintaining high-pressure phases.