Product development research has focused on the action phases of specific projects and relatively ignored the transition between projects. We complement the research on action phases by theorizing and testing a transition model of product development. By building on learning and motivation literature we extend the models of project transitions by theorizing on the factors that link transitions to project performance. We test our model using multi-level structural equation modeling and a longitudinal data set. Finally, although the notion of transition has been recognized as important there has been little empirical testing of its consequences and antecedents. A possible reason for this lack of empirical research is the absence of a valid measure. This study developed and validated the Transition Effectiveness Scale which measures the performance of the transition from one entrepreneurial project into another.