Conceptualization is a critical task in the development of system dynamics models, which starts early in the modeling process, and extends to later stages in the development of any system dynamics project. The procedures and characteristics of model conceptualization have striking parallels with the process of theory building as described in many different strands of literature. Considering also that the modeling process as a whole is an iterative process of comparing and contrasting data and current theories by means of a rigorous yet intuitive process, it seems appropriate to reflect on the modeling process as a theory building effort, which is the main purpose of this paper. In order to illustrate the differences between theory building approaches, the paper presents two examples of system-dynamics-based theory building efforts. Thinking of the model development work as a theory building process has the potential of bringing new insights to the conceptualization of system dynamics models, and to the criteria used to assess the suitability of our models. The paper concludes with the introduction of a set of criteria to assess good theories and with reflections on the further development of these criteria for model validation purposes.