Economies at early stages of development are frequently shaken by large changes in growth rates, whereas advanced economies tend to experience relatively stable growth rates. To explain this pattern, we propose a model of technological diversification. Production makes use of input-varieties that are subject to imperfectly correlated shocks. Endogenous variety adoption by firms raises average productivity and provides diversification benefits against variety-specific shocks. Firm-level and aggregate volatility thus decline as a by-product of the development process. We quantitatively assess the model's predictions and find that it can generate patterns of volatility and development consistent with the data. (JEL D21, D24, E23, O33, O47)