In this first chapter about micromachining with electrochemical discharges, the fundamentals of the material removal mechanism are discussed. Electrochemical discharges provide the energy needed for machining. For a long time it was believed that material removal takes place through melting of the workpiece, similar to electrical discharge machining. It is only quite recently that the important contribution of etching in spark-assisted chemical engraving (SACE) machining, mainly for glass substrates, was recognized. Both of these aspects are discussed in this chapter.