It is a surprising fact that social work is not conceived as a scientific discipline in many countries and especially in the United States. It is surprising because the extent of academic social work programs and the scientific output of people working at schools of social work are significant. And it is surprising anyway if social work is conceived as a profession that by definition is linked to scientific knowledge. This article presents a characterization of the debate on social work science that has been led in the German-speaking countries. As a consequence of this debate and following the philosophy of science as developed by Mario Bunge, it introduces some basic propositions on how to conceive social work as a science, namely as a transdisciplinary action science ("Handlungswissenschaft") and outlines the target of building a consolidated knowledge base of social work as well as the challenges related to this goal.