This Special Issue on Diversified Farming Systems is motivated by a desire to understand how agriculture designed according to whole systems, agroecological principles can contribute to creating a more sustainable, socially just, and secure global food system. We first define Diversified Farming Systems (DFS) as farming practices and landscapes that intentionally include functional biodiversity at multiple spatial and/or temporal scales in order to maintain ecosystem services that provide critical inputs to agriculture, such as soil fertility, pest and disease control, water use efficiency, and pollination. We explore to what extent DFS overlap or are differentiated from existing concepts such as sustainable, multifunctional, organic or ecoagriculture. DFS are components of social-ecological systems that depend on certain combinations of traditional and contemporary knowledge, cultures, practices, and governance structures. Further, as ecosystem services are generated and regenerated within a DFS, the resulting social benefits in turn support the maintenance of the DFS, enhancing its ability to provision these services sustainably. We explore how social institutions, particularly alternative agri-food networks and agrarian movements, may serve to promote DFS approaches, but note that such networks and movements have other primary goals and are not always explicitly connected to the environmental and agroecological concerns embodied within the DFS concept. We examine global trends in agriculture to investigate to what extent industrialized forms of agriculture are replacing former DFS, assess the current and potential contributions of DFS to food security, food sovereignty and the global food supply, and determine where and under what circumstances DFS are expanding rather than contracting.