For more than a decade, the REFSQ community has been acknowledging the difference between ‘method’-focused research (in which methods, techniques, tools – and generally, artefacts, are designed) and evaluation-focused research (in which the fit of proposed artefacts is evaluated in context). Among those researchers developing RE artefacts (i.e. methods/tools/techniques), many have been organizing their research process drawing upon the Design Science Research paradigm. From the perspective of this paradigm, a research project includes three steps: problem analysis, solution design, and solution evaluation. In each of these steps, RE researchers face the choice of using theories. This seminar is about the role of theories and theory-building in research projects that employ Design Science. Participants will learn how the concept of theories could be applied in each of the three steps. Examples will be provided on the application of theories to exploratory research contexts, to RE method design contexts (i.e. design theories), and confirmatory research contexts. Special attention will be paid to validity threats, particularly on the generalizability of results and theories in Design-Science-inspired research.