The first half of this chapter outlines key concepts in cultural-historical activity theory (CHAT), including implicit and explicit mediation and double stimulation. The nature of contradictions within CHAT is described as a springboard for development in workplace practices. The chapter then describes how these concepts have been operationalized in the work of Yrjö Engeström and colleagues through the formative interventionist methodology known as Change Laboratory. Change Laboratories are collaborative settings in which contradictions are brought to conscious awareness and worked on to redesign workplace artefacts, norms, and labor arrangements. As contradictions are resolved through new concretizations of practice, participants in the workplace increase their capacity to develop their working world and, therefore, themselves.