This article conducts a dialogue and creates a new synthesis between two of the most influential ontological discourses in the field of sociology: assemblage theory and critical realism. The former proposes a focus on difference, fluidity, and process, the latter a focus on stability and structure. Drawing on and assessing the work of Deleuze, DeLanda, and Bhaskar, we argue that social ontology must overcome the tendency to bifurcate between these two poles and instead develop an ontology more suited to explaining complex social phenomena by accommodating elements of both traditions. Going beyond DeLanda’s recent work, we argue that a concept of causal types must be used alongside a typology of structures to give us an ontology that can sustain sociology’s need for both formation stories and causation stories. We illustrate the necessity and value of our proposed synthesis by discussing MacKenzie’s recent empirical analysis of a high-frequency trading firm.