The utility of electroanalytical probes for in vivo monitoring of neurotransmitters is derived principally from the small size and high sensitivity of microelectrodes. The high sensitivity of voltammetry compared to optical or magnetic resonance techniques derives from the ability to measure small currents, since a relatively easily measured 1 pA corresponds to ∼10^-17 mol/s of an electroactive molecule. An analytical probe with dimensions of a few micrometers has major advantages in spatial and temporal resolution over a technique requiring extraction or preconcentration of the target molecule from tissue. The ability of a microvoltammetric electrode to detect neurotransmitters in vivo depends not only on sensitivity (signal per unit of neurotransmitter concentration), but also on discrimination of a small analytical signal on top of an often large background from electrode capacitance and/or interfering species in the tissue. Although voltammetry is extraordinarily sensitive, it is not particularly selective and is often hampered by background currents.