Scanning Electro Chemical Microscopy (SECM) is a scanning-probe technique that relies on the measurement of faradaic currents at a microdisk electrode surrounded by an insulating shielding. Typical electrode radii are in the micrometer range. The microelectrode is imersed in an electrolyte solution and scanned in a distance of some electrode radii over the specimen surface. The microelectrode currents orginate from the electrochemical conversion of dissolved species at the microdisk electrode. As the species detected at the microelectrode are formed at the sample surface, the currents at the local probe provide information about the local reactivity of the specimen region directly beneath the microelectrode. The technique can be operated in the generation-collection mode and the feedback mode. Beside mapping local heterogenous reactivities the setup has also been used to induce local surface modifications by local reagent generation at the microelectrode, local pertubation of an equilibrium at the sample surface, or exploitation of the distribution of the electric field strength in the interelectrode space. The explanation of the working principles is illustrated by representative applications in corrosion research, in the study of biochemically active samples, and mass transport phenomena through pores. A concluding section on experimental requirements leads to an outview of possible future developments.