Alternative protein sources for human food, as well as for terrestrial and aquatic animal feed, are increasingly commercially available. These products have considerable potential for sustainably delivering protein for food and feed and could lead to significant reductions in climate and land use impacts. Alternative protein sources include meat analogs, insects, certain woody plants, and algae including seaweed. We briefly review recent work on their nutritional, environmental, technological, and socioeconomic impacts. We outline possible trade-offs at scale and consider the ways in which the alternative protein sector may develop in the next decades.