In its simplest manifestation, the electrochemical quartz crystal microbalance (EQCM) is a relatively new device for executing a classical technique, electrogravimetry. The advantages it brought were in situ applicability (notwithstanding prior misconceptions regarding damping by a contacting fluid), exceptional sensitivity and dynamic capability, thereby permitting real-time monitoring of changes in surface populations of species during electrochemically driven processes. The basis of the method relies on the storage and dissipation of energy injected into the interfacial region by a high frequency (megahertz) acoustic wave; the latter is generated by a piezoelectric (generally quartz) resonator. From modest early aspirations, largely associated with the deposition/dissolution of simple adsorbates and thin metal films, the technique has expanded in three strategic respects: materials, phenomena and methodology. In the first instance, extension to thick electroactive films (notably metal oxides and polymers) has generated considerable insight. Second, the sensitivity of the EQCM to viscoelastic phenomena, stress and interfacial slip has been recognized. Considerable attention has been given to viscoelastic processes in redox and conducting polymers: these have been parameterized in terms of shear moduli, whose variation with polymer structure and imposed conditions provides insight into polymer dynamics. Procedures exist for characterizing film stress in harder materials, but this is less well exploited. Interfacial slip remains a poorly understood area. Third, application in the context of diverse electrochemical control functions and integration with other in situ techniques provide many as yet unexploited opportunities. The extent to which these are realised will probably depend on the level of interpretation of the resultant data, which presently underuses the library of modelling protocols available.