Black-hole masses of distant quasars cannot be measured directly, but can be estimated to within a factor 3 to 5 using scaling relationships involving the quasar luminosity and broad-line width. Why such relationships are reasonable is summarized. The results of applying scaling relationships to data of quasars at a range of redshifts (z ≤ 6.3) are presented. Luminous quasars typically have masses ~ 109M⊙ even at the highest redshifts. The fact that such massive black holes appear as early as at z ≈ 6 indicate that black holes form very early or build up mass very fast.