This paper uses Vygotsky's cultural-historical development theory to examine the theoretical lineages that influenced Marsick and Watkins' (1990) model of informal and incidental learning. After discussing the context of each influence, the paper applies cultural-historical development theory to the many updates that Marsick, Watkins, and collaborators have contributed, moving the model into a theory. In 1990, Marsick and Watkins named the work of Dewey, Lindeman, Lewin, Knowles, Polanyi, and Argyris and Schön as primary influences for the formation of their informal and incidental learning model. By focusing on the context of each of the theorists who influenced the original Marsick and Watkins model and sketching the connections between the theorists' ideas and experiences, this paper fulfills the twofold purpose of exploring the context of Marsick and Watkins' model-turned-theory and providing a model for Vygotsky-informed literature reviews.