The basic principle of electrochemical machining (ECM) is the removal or surface modification of the anodically polarized workpiece caused by electrolysis. First, the basic reduction and oxidation processes (redox processes) are described, which occur when a current passes through an electrolyte and are specifically initiated by the positioning of a cathodically polarized tool electrode in ECM. This results in Joule heating of the electrolyte, which significantly influences the local removal characteristics. The most important ECM machine parameters, pulsed ECM technologies (pulse ECM, PECM), commercially available electrolytes and tool and workpiece materials are introduced. The application of established heuristic and more recent numerical methods for the design of cathode geometries in ECM is also discussed in detail. Numerous practical examples are used to demonstrate the efficiency of process modifications in ECM machining. In a separate section, plasma-assisted anodic oxidation processes for surface preservation are discussed. Here it is shown that by selecting suitable electrolytes and current parameters, plasma discharges can also be used to build up new surfaces with specific properties instead of removing them.