This chapter provides a brief analysis of the main perspectives in effectiveness research in the context of the complexities of social work practice, and the contribution single-case evaluation can make within each of the perspectives. It explains two main purposes of effectiveness practice research——providing evidence of the worth of social work practice and striving to improve practice itself to respond to the changing needs and contexts. Social work practice takes place in an open system, usually working with the social work client in a holistic way. There are four main perspectives that influence social work practice research: empirical practice, pragmatism or methodological-pluralism, critical theory and scientific realism. Empirical practice tends to be associated with positivism and it is within this perspective that single-case-evaluation as a methodology was boom. An important contribution of scientific realism is that it is inclusive of all the other realist perspectives including empirical practice, pragmatism and critical theory.