Domain-independent models of the design process are an important means for facilitating interdisciplinary communication and for supporting multidisciplinary design. Many so-called domain-independent models are, however, not really domain independent. We state that to be domain independent, the models must abstract from domain-specific aspects, be based on the study of several design disciplines, and be useful for many design disciplines and for multidisciplinary design teams. This paper presents a domain-independent descriptive design model that is developed by studying similarities and differences between design processes in three design disciplines and which is based on the general theory of state-transition systems. The main concepts of the descriptive model are a design situation and a design activity. The descriptive model is applied in a domain-independent prescriptive model that supports designers with structured reflection on a design process; reflection provides an important possibility in improving the effectiveness and efficiency of design processes.