Microelectrodes, defined in this chapter as electrodes of micrometer or smaller dimensions, have received considerable attention since the early 1980s in investigations by electroanalytical chemists. What can be done with these small electrodes? It should be obvious that they are not very useful for the generation of large amounts of material. On the other hand, it is clear that their small size should make them useful to probe the chemical composition of electroactive species in small spaces. Indeed, the use of true microelectrodes for amperometry was pioneered in the 1940s to measure oxygen concentrations inside biological tissues [1]. Today, microelectrodes have been developed that can be used inside the living brain to probe the dynamic concentrations of neurotransmitters, chemical substances that relay information between neurons [2]. Voltammetric electrodes can be fabricated with such a small size that they can actually be used to probe chemical events inside single biological cells [3].