Carbon-fiber microelectrodes have been the fundamental tool for the electrochemical detection of neurotransmitters since their introduction in the early 1980s. They are biologically compatible and, compared to larger electrodes, microelectrodes offer highly increased diffusional flux to the electrode surface and faster response times to enable high speed measurements. The demonstrated benefit of enhanced mass transport to very small electrodes has led us to develop several new carbon-based microelectrodes for the improved detection of dynamic neurochemical fluctuations. This talk will describe the characterization of a recessed microdisk electrode for enhanced detection of individual exocytosis events at single cells, a nanoscale microelectrode with a conical geometry for fast intracellular voltammetric measurements, and innovative yarn-like sensing substrates engineered entirely of multi-walled carbon nanotubes for enhanced sensitivity when monitoring from individual cells or discrete brain regions. Each of these ultramicroelectrode designs exploits mass transport to either provide maximal diffusional flux to, or to limit diffusional flux from, the electrode surface. These new tools provide significant advantages when coupled with fast-scan cyclic voltammetry, and applications that provide new insight into neurochemical dynamics will be discussed.