This article adds a new case study to the expanding literature on socio-technical transitions: the shift from mixed farming to intensive pig husbandry in the Netherlands. With regard to this transition, the article addresses a new direction, namely the role of foundational ontologies in explanation. Five ontologies are distinguished, which are based on different assumptions about causal agents and causal mechanisms: rational choice, functionalism, conflict and power struggle, interpretivism, and structuralism. The article demonstrates how these ontologies provide different explanations of same case. It also empirically investigates the strengths and weaknesses of different ontological explanations, and identifies possible complementarities in this case. The article ends with theoretical reflections on the relationships between ontologies and the role of meta-paradigm analysis. © 2009 Taylor & Francis.