This chapter discusses structures and agency, the morphogenetic nature of their interaction (Archer, 1998) and the implications of the stratification of living things for agency. A critical realist approach allows us to study structure and agency across the sometimes jealously guarded boundaries of traditional disciplines because we are led by an exploration of the ontology of the research area across disciplinary traditions. Along the way we shall find ourselves confronting two recurrent themes. First, there is the materialism/idealism debate and second, the reductionism/holism debate. These themes resurface across disciplines. This chapter ends with an elaboration of what a stratified ontology of human life and behaviour means. The next section defines and elaborates on the concept of structure.