This paper reports on the results of a study that investigates how design principles are used in design practice. Design principles have become the predominant way to capture abstract knowledge about the design of information systems (IS) artifacts—and as design science researchers, we expect that practitioners will use these outcomes of our work. Our empirical evidence is drawn from the analysis of spoken-out thought processes of designers as they attempted to use a certain set of design principles in a new context. Through our analysis, we identify five key categories conceptualizing the use of design principles: interpreting scope and content, matching with problem space, guesstimating missing information, projecting into solution space, and implanting into design process. We find that design principles do not shut down degrees of freedom, but rather, channel actions from the designer, who acts in a conscious, deliberative manner to creatively apply the design principles. Through our work, we contribute to (a) our understanding of how design principles are incorporated in design processes, and (b) the emergent stream of research about the formulation of design principles.