Nano and micro electro-mechanical systems (NEMS and MEMS) have been attracting a large amount of attention recently as they have extensive current/potential applications. However, due to their scale, molecular interaction and size effects are considerably high which needs to be considered in the theoretical modelling of their electro-mechanical behaviour. Both nano- and micro-scale electrically actuated structures are discussed when subjected to constant and time-varying voltages, and different theories and models, introduced in the past few years for modelling such small structures, are discussed. It is highlighted that considering the intermolecular forces and size-dependence effects can change both the static and dynamic behaviours of such systems significantly. This review presents the current stage of the research on electrically actuated NEMS/MEMS by analysing the latest models and studies in this field in the framework of electro-mechanical coupling and small-size effects.