Agroecology can be defined as scientific research on ecological sustainability of food systems. In addressing food production and consumption systems in their entirety, the focus of agroecology is on interactions and processes that are relevant for transitioning and maintaining ecological, economic, political, and social-cultural sustainability. As a field of sustainability science, agroecology explores agriculture and food with explicit linkages to two other widespread interpretations of the concept of agroecology: environmentally sound farming practices and social movements for food security and food sovereignty. In the study of agroecology as science, both farming practices and social movements emerge as integrated components of agroecological research and development. In the context of agroecology, the concept of ecology refers not only to the science of ecology as biological research but also to environmental and social sciences with research on social systems as integrated social and ecological systems. In agroecological theory, all these three are merged so that agroecology can broadly be defined as “human food ecology” or “the ecology of food systems.” Since the last decades of the 20th century many developments have led to an increased emphasis on agroecology in universities, nonprofit organizations, movements, government programs, and the United Nations. All of these have raised a growing attention to ecological, environmental, and social dimensions of farming and food, and to the question of how to make the transition to sustainable farming and food systems. One part of the foundation of agroecology was built during the 1960s when ecologically oriented environmental research on agriculture emerged as the era of optimism about component research began to erode. Largely, this took place as a reaction to unexpected and unwanted ecological and social consequences of the Green Revolution, a post–World War II scaling-up, chemicalization, and mechanization of agriculture. Another part of the foundation was a nongovernmental movement among thoughtful farmers wanting to develop sustainable and more ecological/organic ways of production and the demand by consumers for such food products. Finally, a greater societal acceptance, demand for research and education, and public funding for not only environmental ecology but also for wider sustainability in food and agriculture was ignited by an almost sudden high-level political awakening to the need for sustainable development by the end of 1980s. Agroecology as science evolved from early studies on agricultural ecosystems, from research agendas for environmentally sound farming practices, and from concerns about addressing wider sustainability; all these shared several forms of systems thinking. In universities and research institutions, agroecologists most often work in faculties of food and agriculture. They share resources and projects among scientists having disciplinary backgrounds in genetics (breeding of plants and animals), physiology (crop science, animal husbandry, human nutrition), microbiology or entomology (crop protection), chemistry and physics (soil science, agricultural and food chemistry, agricultural and food technology), economics (of agriculture and food systems), marketing, behavioral sciences (consumer studies), and policy research (agricultural and food policy). While agroecologists clearly have a mandate to address ecology of farmland, its biodiversity, and ecosystem services, one of the greatest added values from agroecology in research communities comes from its wider systems approach. Agroecologists complement reductionist research programs where scientists seek more detailed understanding of detail and mechanisms and put these into context by developing a broader appreciation of the whole. Those in agroecology integrate results from disciplinary research and increase relevance and adoption by introducing transdisciplinarity, co-creation of information and practices, together with other actors in the system. Agroecology is the field in sustainability science that is devoted to food system transformation and resilience. Agroecology uses the concept of “agroecosystem” in broad ecological and social terms and uses these at multiple scales, from fields to farms to farming landscapes and regions. Food systems depend on functioning agroecosystems as one of their subsystems, and all the subsystems of a food system interact through positive and negative feedbacks, in their complex biophysical, sociocultural, and economic dimensions. In embracing wholeness and connectivity, proponents of agroecology focus on the uniqueness of each place and food system, as well as solutions appropriate to their resources and constraints.