•Key principles of self-assembly at electrified solid–liquid interfaces are reviewed.•The effect of the electric field in the double layer on intermolecular and substrate–molecule interactions is explained.•The equivalence between the electrochemical interface and the field in an STM configuration is analyzed. Supramolecular chemistry at solid–liquid interfaces is guided by the interactions between the molecular building blocks, the solid substrate and the liquid phase. At an electrochemical interface (i.e., at the interface between an electronic and ionic conductor), the substrate potential allows modulating many of these interactions, resulting in a high level of control over the supramolecular structures that are formed and their reactivity. In this paper, we review key principles and recent work in this area, and discuss how a standard scanning tunneling microscope setup allows to scale down interfacial supramolecular electrochemistry to the few-molecule level.