The capital asset pricing model (CAPM) builds on the Markowitz mean-variance-efficiency model in which risk-averse investors with a one-period horizon care only about expected returns and the variance of returns (risk). These investors choose only efficient port-folios with minimum variance, given expected return, and maximum expected return, given variance. Expected returns and variance plot a parabola, and points above its global minimum identify a mean-variance-efficient frontier of risky assets.