This thesis is a treatise on a theory for societal transitions: pillar theory. Societal transitions are complex processes taking place in complex systems, large-scale, long-term processes in which societal systems radically change the way they are composed and function. Since we all are part of societal systems, it speaks for itself that we ought to want to understand transitions. Nevertheless, although several aspects of transitions have been studied from various perspectives in various disciplines, the study of societal transitions as such is a relatively recent development. Consequently, the knowledge on transitions is scattered over disciplines and rather fragmented. Understanding requires theory, for even articulating what one doesn't know about a certain subject inevitably requires phrasing it as a question using concepts. Theory is an intellectual tool. It also works the other way around. Theory is also the result of understanding and it is this Janus-faced property on which this thesis is based.