Electrochemical scanning probe microscopy (SPM) has evolved from its initial inception in about 1989 to encompass a range of techniques providing unique information on the reactivity of surfaces, molecular and ionic diffusion, the kinetics of dynamic processes, and topographical imaging at a highly localised scale ranging from nm to µm. Electrochemical-SPM includes scanning electrochemical microscopy, electrochemical atomic force microscopy (AFM), electrochemical scanning tunnelling microscopy (STM), scanning vibrating electrode technique, scanning Kelvin probe, scanning Kelvin probe force microscopy, and scanning ion conductance microscopy. Investigations using these varied techniques include molecular imaging, kinetics of enzyme catalysis, kinetics of homogeneous reactions and of heterogeneous reaction (corrosion, catalysis), imaging of molecular transport through pores in polymer and biological membranes, quantification of fluxes though pores, imaging in thin-liquid layers, charge transfer at immiscible liquid-liquid interfaces, micro- and nanopatterning, electrochemical switching of molecular wires, and sensors. The range of innovative applications has continued to expand as the potential of electrochemical- SPM has become increasingly recognised. This overview describes the principle of the techniques and highlights the many exciting applications and the innovative ways that the technique has been adapted. Future developments envisage smaller probes and the combination of electrochemical scanning probe microscopy with complementary techniques such as near-field scanning optical microscopy (NSOM). NPL has only recently started to explore the application of electrochemical-SPM techniques. It is essential that expertise in this field is developed in support of measurements in materials, biological systems, and analytical chemistry. Currently, there are no measurement standards in electrochemical-SPM and NPL could play a key role in supporting their development.