In this paper we take a portfolio approach to analyze the investment strategy of a venture capitalist (VC), and study the optimal size of a VC's portfolio. We show that portfolio size and scope a¤ect the incentives of both entrepreneurs to exert e¤ort and VCs to make start-up speci…c investment. A small portfolio improves entrepreneurial incentives because it allows the VC to concentrate his limited human capital on a smaller number of start-ups, adding more value to each start-up. In addition, by holding a small portfolio, the VC commits not to extract higher rents from the entrepreneurs, with a positive impact on their incentives. A large and focused portfolio is bene…cial for the VC because it allows the VC to reallocate his limited resources and human capital from one start-up to another in case of a startup failure. Furthermore, a large and focused portfolio allows the VC to extract greater rents from the start-ups because the VC can induce competition for his limited resources. We show that the VC …nds it optimal to limit his portfolio size when start-ups have higher quality prospects ex ante, that is, when providing strong entrepreneurial incentives is most valuable. The VC expands his portfolio size only when start-up fundamentals are more moderate and only when he can form a su¢ ciently focused portfolio.Finally we show that the VC may …nd it optimal to engage in portfolio management by divesting some of his start-ups early since this strategy allows him to extract a greater surplus from the remaining start-ups in his portfolio. Surprisingly, we …nd that under certain conditions, portfolio management, despite being socially ine¢ cient ex post, improves ex ante social welfare by enlarging the set of economically viable start-ups.