In this chapter, I will identify different categories of theoretical perspectives and judge how they are useful for the study of public service delivery by the third sector. The perspectives will be illustrated with interesting examples from current literature. The specific way in which I frame them is borrowed from the work of the sociologist Charles Tilly, which I find particularly perceptive. However, the purpose of the chapter is not to argue that his way of framing different types of theories is superior to all others. There are different, equally valid ways to categorise them. The point is that applying such a framework can be a useful exercise for understanding the relative strengths of different theoretical approaches in a particular field of research. The premise is that each perspective highlights certain aspects of reality at the expense of others. Consequently, if the object of an empirical investigation is to examine a specific aspect of a phenomenon, it makes sense to analyse it with a theoretical approach that is likely to deliver the best results. The question posed in the chapter is: how does this work for public service delivery by the third sector?