This paper is about the development of design theory, about how research in design theory is — and should be — done. Important works in design theory since 1850 are identified. The current state of design as a pre-paradigm science is explained, motivating the need for a unifying view of design. Given the state of the field, a framework for design theory is presented, and a research process model is proposed as a scientific way of doing research in design theory. These are discussed in light of modern views in the philosophy of science and their criteria for theory evaluation and scientific progress. Research in design can be treated in a scientific manner, but this must be done from a sophisticated view of scientific practice, one with the proper criteria for evaluating progress: at the level of the research program.