Set of portfolios with the minimum variance for each level of expected return — the upper boundary of the feasible mean-variance region. With a risk-free asset, the frontier becomes the Capital Market Line E[R_p] = r_f + dfracE[R_T] - r_fσ_T,σ_p, tangent to the risky-asset frontier at the tangency portfolio T, and the Sharpe ratio is maximised at T. The frontier is notoriously fragile in-sample: estimation error in boldsymbolμ and boldsymbolΣ produces corner-solution weights, motivating Black-Litterman, shrinkage, and resampled-efficiency fixes.