Ultramicroelectrodes offer several unique characteristics which enable new types of electrochemical measurements. These include: 1) small size; 2) minimisation of iR effects; 3) rapid response; and 4) steady-state response at moderate times. These features enable experiments as diverse as in vivo electrochemistry, electrochemistry in pharmacology, nanoelectrochemistry, electrochemistry in solvents such as benzene, microsecond electrochemistry, and flow-rate independent electrochemistry. Thus, it is apparent that the use of ultramicroelectrodes has become a rapidly growing area of interest. In this paper, the attributes of ultramicroelectrodes, its construction, the equations of diffusion, and key applications of electrochemistry at ultramicroelectrodes, are analysed.