Using recent criticisms and suggestions regarding the multi-level perspective as stepping stones, the article aims to enhance the reflexivity in transition debates regarding social theories. To that end, the article discusses seven social science ontologies (rational choice, evolution theory, structuralism, interpretivism, functionalism, conflict and power struggle, relationism), their assumptions on agency and causal mechanisms, and their views on socio-technical transitions and environmental sustainability. The second goal is to position the multi-level perspective on transitions with regard to these ontologies and to identify directions for theoretical extensions. The MLP is characterized not as a grand or unifying theory, but as a middle range theory that makes crossovers to some ontologies and not to others. © 2010 Elsevier B.V. All rights reserved.