Ternary organic solar cells (TSCs) contain a single three-component photoactive layer with a wide absorption window, which is obtained without the need for multiple stacks. Subsequently, TSCs have attracted great interest in the photovoltaics field. Through careful selection of the three (or more) active components that form the photoactive layer, all photovoltaic parameters can be simultaneously enhanced within a TSC — a strategy that has resulted in record efficiencies for single-junction solar cells. In this Review, we outline key developments in TSCs, with a focus on the central role of the third component in achieving record efficiencies. We analyse the effects of the third component on the nanomorphology of the bulk heterojunction and the photovoltaic parameters of TSCs. Moreover, we discuss the charge-transfer and/or energy-transfer mechanisms and nanomorphology models that govern the operation of TSCs. We consider both polymer and small-molecule donors as well as fullerenes and recently developed non-fullerene acceptors. In addition, we summarize the recent success of TSCs in mitigating the stability issues of binary solar cells. Finally, we provide a perspective on the advantages of ternary blends and suggest design strategies for highly efficient and stable devices for commercial photovoltaics.