Within the information systems community there is growing interest in design theories. These theories are aimed to give knowledge support to design activities. Design theories are considered as theorized practical knowledge. This paper is an inquiry into the epistemology of design theories. It is an inquiry in how to justify such knowledge; the need to ground and how to ground a design theory. A distinction is made between empirical, theoretical and internal grounding. The empirical grounding has to do with the effectiveness of the application of knowledge. External theoretical grounding relates design theory to other theories. One part of this is the grounding of the design knowledge in general explanatory theories. Internal grounding means an investigation of internal warrants (e.g. as values and categories) and internal cohesion of the knowledge. Together, these different grounding processes form a coherent approach for the multi-grounding of design theory (MGDT). As illustrations some examples of design theories in IS are discussed. These are design theories concerning business interaction which are based on language action theories.