With today's requirements for miniaturization and high efficiency in mind, the author describes the most up-to-date power supply systems. Part A describes, from a practical point of view, regulated linear and unregulated DC-DC converter power supplies providing stabilized output voltages. With the spread of alternative battery-powered appliances, the latter power supplies are steadily gaining in importance. This part also contains a detailed glossary of technical terms. To facilitate the use of catalogue data sheets, the internationally accepted terms, definitions and typical parameters are listed, and the technical parameters to be specified are summarized. Part B describes the latest special power supplies: the switched-mode power supplies and regulated DC-DC converters to be operated from a power line or battery. In view of the pressure for miniaturization, their small masses and volumes as well as their high efficiency and energy-storage capacity make them superior to conventional arrangements. These recent circuit versions, which today operate even at 200 kHz, comply with the requirements of the present energy-saving era and are beginning to find a market.