For many years, quantitative investors trying to balance risk and return have been guided by academic finance. Harry Markowitz taught us to think about portfolios rather than individual securities. Most of his work focuses on static, or single-period, assessment of the trade-off between the mean and variance of an expected portfolio return distribution. His 1950s innovation was followed in the 1960s by the capital asset pricing model (CAPM), articulated most convincingly by William F. Sharpe. CAPM taught us the value of index funds.