The technique of scanning electrochemical microscopy (SECM) is described. In this technique the electrolysis current that flows as an ultramicroelectrode tip (diameter ca. 10 μm) immersed in a solution is moved above a substrate surface is used to characterize processes and structural features of the substrate. Modes of operation considered include collection modes, where products electrogenerated at the substrate are detected at the tip (held at constant potential or operated in the cyclic voltammetric mode), and feedback modes, where the effect of substrate on the tip current is monitored. The feedback mode can be used with both conductive and insulating substrates and is less sensitive to electrical coupling between substrate and tip. An alternating current generation/collection mode is also described. Experimental results for the different modes of operation and proposed extensions of the SECM technique are presented.