Dr. MacLeod's empirical and theoretical contributions include some of the most striking advances in vision science in the second half of the twentieth century. Among his many contributions, MacLeod (with Boynton, 1979) developed a constant luminance cone excitation chromaticity space. The MacLeod-Boynton chromaticity diagram is widely used, and has become the preferred way of expressing chromatic discrimination data. The Commission Internationale de l'Eclairage will include a version of the MacLeod-Boynton chromaticity diagram in its new physiologically based colorimetric system. With Eisner (1980), he provided experimental evidence that the S-cones do not contribute to flicker photometric sensitivity. With Williams and Hayhoe (1981), MacLeod psychophysically mapped the S-cone distribution in the fovea. With Webster (1983, 1987), MacLeod analyzed individual differences in cone receptoral spectral sensitivities before molecular genetics provided a cellular basis fori functional polymorphism. The conclusions of the Webster-MacLeod analysis are concordant with modern molecular genetic studies.