The electrode is a vital element of a microelectrode biosensor, which is designed to transform the recognition of a biological molecule into an easily quantifiable electrical signal. The most widely used enzymatic biosensors rely on oxidases for detecting neurotransmitters and metabolites in the central nervous system. These enzymes oxidize their substrates while producing peroxide (H2 O2 ) from molecular oxygen (O2 ). H2 O2 is then oxidized on the microelectrode to produce an oxidation current that is directly proportional to the analyte concentration and can readily be measured by a low-current potentiostat. Therefore, the microelectrode serves both as a substrate to immobilize the enzyme and as a material to catalyze H2 O2 oxidation and pass electrical current. In this chapter, we review two major types of microelectrode design. For many years, neuroscientists have used handmade wire electrodes consisting of a platinum (or carbon) wire inserted into a pulled glass capillary. Within the last 10 years, exciting new methods relying on microsystem technologies have emerged. Using these processes, microprobes with multiple integrated microelectrodes can be generated on a semi-industrial scale allowing simultaneous detection of several molecules of interest in different brain areas. Wire electrodes, which can reach extremely small sizes, could remain useful for specific neuroscience applications requiring very high spatial resolution; however, microfabricated silicon needles will undoubtedly contribute to future developments in the field of microelectrode biosensors allowing multi-analyte and network-oriented neuronal monitoring.