The growing volumes of food globally lost or wasted and implications for food security and sustainability have raised the concern of researchers, governments, international organizations and grass-root movements. Much research and experiences investigating food waste causes and drivers focus on one specific segment of the food supply chain and limit the analysis to the situation of one or few countries, while the few studies of wider geographical scope also target other relevant and diversified objectives (e.g., food waste definition, quantification, environmental and economic impacts, and recommendations for interventions). This study, carried out by a network of European institutions involved in research and initiatives against food waste, focuses on the analysis of a broad area, Europe, through a wide and systematic literature review and consultation with stakeholders in international focus groups. The food supply chain was divided into seven segments and three main contexts were defined for the examination of food waste sources: Technological, Institutional (related to organisational factors, i.e., business management, economy, legislation, and policy), and Social (related to consumers' behaviours and lifestyles). Results suggest a wide and multifaceted problem, interconnected across all stages of the food supply chain, from primary production, to final consumption. Within each context, the identified drivers have been grouped according to the possibilities and the type of interventions for food waste reduction. A final cross-contextual prioritization distinguished food waste sources related to (A) inherent characteristics of food; (B) social and economic factors; (C) individual non-readily changeable behaviours; (D) other priorities targeted by private and public stakeholders; (E) diversified factors, such as mismanagement, inefficient legislation, lack of awareness or information; and sub-optimal use of available technologies, which could be more promptly changed. Such diversification of causes calls for specific monitoring systems, targeted policy measures, and actions of individual stakeholders at each stage of the food supply chain.