The partial shading phenomenon and its implications on the electrical response and energy yield of photovoltaic (PV) systems have received increased attention in the last years. In order to study, foresee and mitigate such effects, several energy models are proposed in the bibliography, presenting different degrees of complexity, accuracy and applicability. This study presents an overview of the state of the art in the development of models for PV systems under partial shading conditions. Alternative modelling approaches are analysed, highlighting their advantages and shortcomings and models available in the literature are reviewed and classified according to important attributes, related to their accuracy and implementability. Current research trends, as well as topics that warrant further investigation, are identified and discussed.