Spark-assisted chemical engraving (SACE) is a promising micro-machining technology for the low-cost machining of holes and channels in non-conducting materials, such as glass and some ceramics. Despite the complexity of SACE due to the interdependency of thermal, electrochemical and mechanical effects, the key data of the machining process can be obtained from only a few signals. Possible process surveillance signals are analysed and discussed. In particular, the current flowing between the electrodes is analysed. It is shown that various information can be deduced from it, such as qualitative indications of the local electrolyte temperature, differentiation between machining and non-machining and the distinction between the presence and absence of a gas film. However, so far no direct link between the current and the instantaneous material removal rate could be found. Experimental results are presented and the possibility of active process control based on current measurement is discussed.