System dynamics requires the intense use of qualitative data and human judgment in all stages of model development. Most approaches to the formal inclusion of qualitative data have been developed with the purposes of knowledge elicitation during the conceptualization or formulation stages of model development. Although the importance of using expert judgment to assess the validity of system dynamics models is well recognized, the development of approaches to use this kind of judgment is not well developed. In recent years, efforts to develop tools to assess the validity of system dynamics models by interviewing experts have been explored in some doctoral work. This paper reviews the basic concepts of model validation, and explores the use of interviews as a research and knowledge-acquisition technique. Finally, it documents and compares four applications of interviewing as a tool to assess system dynamics models, ending with recommendations for both the practitioner and researcher.