This paper offers a theory of development that links the degree of market incompleteness to capital accumulation and growth. Because sectoral indivisibilities limit the extent of diversification, poor economies suffer higher volatility of growth and endogenously lower productivity. As the economy develops, agents hold more balanced portfolios and can take better advantage of high-return production opportunities. Although all agents are price takers and there are no technological spillovers, the decentralized equilibrium is inefficient because individuals do not internalize the impact of their investment decisions on others' diversification opportunities. The results generalize to economies with international capital flows. Copyright 1997 by the University of Chicago.