Food systems exert major pressures on the environment. This paper reviews what is known and not known about environmental impacts along food supply chains, looking at the contribution of different stages of the supply chain, the impact of different products, heterogeneity among producers, and the role of international trade. This review shows that most environmental impacts in food supply chains occur through land use change or at the stage of agricultural production. Livestock (especially ruminant livestock) has a higher footprint than plant-based food. However, there is also important heterogeneity among producers, even within the same region. A significant share of total environmental impacts is "embodied" in international trade, although considerably less than half. In terms of evidence gaps, some impacts (e.g. biodiversity, soil carbon) have been less studied, and there are geographic and product blind spots. Moreover, existing evidence is not sufficiently granular. While important evidence gaps thus exist, the overall picture that emerges is one of a rapidly growing evidence base, which can inform innovative supply chain initiatives to reduce impacts.