How to approach social theory actively, with a theoretical lens analogous to the use of methods, has been a challenge for professional scholars and students alike. Rather than treating social theory in an iconic manner, we explore the active use of theorizing for constructing and generating new knowledge. Examples of theoretical constructions and topics discussed include: the heuristic role of concepts; theoretical construction work; the importance of question-driven sociology; counterfactual reasoning; the power of ordinary language; an inventory of explanatory practices in social science; abduction; comparative case studies; class operations and the potential for using Merton’s middle range theory. Theory in Action is highly relevant for researchers and students interested in constructing theories in the social sciences. Contributors are: Göran Ahrne, Mette Andersson, Roar Hagen, Willy Guneriussen, Ragnvald Kalleberg, Håkon Leiulfsrud, Willy Martinussen, Annick Prieur, Peter Sohlberg, Pål Strandbakken, and Richard Swedberg.