<p>Though the concept of differentiation has long been central to sociological theory, efforts to build a cumulative body of knowledge about differentiation dynamics are currently hindered by a lack of consensus about the way we should refer to certain processes and states. Addressing this issue, the following article provides an analysis of terminological issues in differentiation theory, highlighting points of weakness in our current conceptual palette. By charting the varied meanings which scholars have attached to notions of “social,” “structural,” and “functional” differentiation, I show that theorists readily use these terms, or make distinctions between different types of differentiation, in ways which are ambiguous, idiosyncratic, or incompatible with existing theory. Ultimately, the piece argues that a more consistent terminology, and a greater use of more specific and targeted terms, will help us avoid some common points of confusion, and will allow us to theorize differentiation processes in a more coherent and communicable manner.</p>