The development of the power electronics in the last twenty years (in connection with the microelectronic) was the basis for the great progress of the drive controls, the energy transmission and for all fields of the electric energy management. After a short overview of the historical development of the power electronics, the basic uses – the switch of current and voltage – and the resulting effects on net and consumer are presented (EMC). Next, the most important power-semiconductor devices and their characteristics are discussed: diodes, bipolar and MOS-transistors, IGBTs, thyristors, triacs, GTO, MCT, Sith. This will be completed by the functionality of protection circuits. The next topics are about power converters for DC drives as controlled line-commutated bridges, converters, direct cyclo-converters and self-commutated four-quadrant controllers. The third part deals with power converters for AC drives with voltage and current converters. Finally, the functioning of the converter as actuators and control elements in the drive engineering is discussed. Furthermore an outlook to the applications of converters e.g. in machine tools (industrial drives) and tractions drives is given.