["Journal of Economics &Management Strategy, EarlyView. ", "\nABSTRACT\nThis paper presents a model of vertical market structure that integrates multiple determinants previously studied in isolation. It shows how the nature of market growth, whether through existing or new applications of an input, and the type of fixed costs, whether exogenous or endogenous, affect the emergence of external suppliers for the input. The role of such suppliers depends on the input's technological generalizability, its potential for improvement, and the breadth and differentiation among downstream applications. Together, these factors shape whether a vertical market structure persists and can potentially drive multiple transitions between vertical integration and outsourcing.\n"]