The common thread binding all electroanalytical methods is heterogeneity. The very act of placing an electrode in contact with a solution creates a phase boundary that differentiates otherwise identical solute molecules into two types: those at a distance from the electrode, and those close enough to participate in the fascinating mutual interactions known collectively as electrochemistry. This is not a trivial distinction, for often it is the bulk-phase properties alone that are of analytical concern. Unlike most spectroscopic methods, electrochemical measurements are actually made on only a minute fraction of the sample confined to a highly nonhomogeneous environment, the electrode-solution interphase. The coupling and interplay within this region of such phenomena as interfacial charge transfer, diffusional mass transport, adsorption, chemisorption, homogeneous-phase chemical reaction, convection, and dissolution can cloud the interpretation of electrochemical data and discourage the practically minded analyst. On the other hand, electrochemistry offers an invaluable tool for fundamental investigation of these processes, each important in its own right. In either case, the ultimate success of the experimenter will depend on a firm grasp of the underlying physical principles.