A unified model for the evolution of galaxies and quasars incorporates a simple scheme for the growth of supermassive black holes into semi-analytic models that follow the formation and evolution of galaxies in a cold dark matter-dominated Universe. The model assumes that supermassive black holes are formed and fueled during major mergers. When two galaxies of comparable mass merge, their central black holes coalesce, and a few percent of the gas in the merger remnant is accreted by the new black hole over a time-scale of a few times 10^7 yr. This model not only fits many aspects of the observed evolution of galaxies but also reproduces the observed relation between bulge luminosity and black hole mass, the strong evolution of the quasar population with redshift, and the relation between the luminosities of nearby quasars and their host galaxies. The decline in the number density of quasars from z ~ 2 to z = 0 is due to a decrease in the merging rate, a decrease in the amount of cold gas available to fuel black holes, and an increase in the time-scale for gas accretion. The predicted decline in the total content of cold gas in galaxies is consistent with observations of damped Lyα systems. The results suggest that the evolution of supermassive black holes, quasars, and starburst galaxies is closely linked to the hierarchical build-up of galaxies. The model uses semi-analytic techniques to simulate galaxy formation, including the merging history of dark matter haloes, cooling, star formation, and feedback cycles. The model shows that the evolution of the quasar luminosity function with redshift is well explained by the model, and that the observed evolution of the quasar population is consistent with the hierarchical build-up of galaxies. The model also fits the observed evolution of the star formation rate density, the total mass density in cold gas, and the luminosity function of galaxies. The model predicts that the number density of quasars declines with redshift due to the combination of three effects: a decrease in the merging rate, a decrease in the amount of cold gas available to fuel black holes, and an increase in the time-scale for gas accretion. The model also shows that the evolution of the quasar luminosity function is closely related to the evolution of the cold gas content in galaxies. The model is tested against observational data and shows good agreement with the observed evolution of the quasar population and the cold gas content in galaxies. The model also predicts that the luminosity of quasars declines with time after the merging event, and that the ratio of quasar luminosity to Eddington luminosity increases with redshift. The model is sensitive to assumptions about cooling, star formation, and accretion time-scales, and shows that the evolution of the quasar population is strongly influenced by these factors. The model also predicts that the host galaxies of quasars areA unified model for the evolution of galaxies and quasars incorporates a simple scheme for the growth of supermassive black holes into semi-analytic models that follow the formation and evolution of galaxies in a cold dark matter-dominated Universe. The model assumes that supermassive black holes are formed and fueled during major mergers. When two galaxies of comparable mass merge, their central black holes coalesce, and a few percent of the gas in the merger remnant is accreted by the new black hole over a time-scale of a few times 10^7 yr. This model not only fits many aspects of the observed evolution of galaxies but also reproduces the observed relation between bulge luminosity and black hole mass, the strong evolution of the quasar population with redshift, and the relation between the luminosities of nearby quasars and their host galaxies. The decline in the number density of quasars from z ~ 2 to z = 0 is due to a decrease in the merging rate, a decrease in the amount of cold gas available to fuel black holes, and an increase in the time-scale for gas accretion. The predicted decline in the total content of cold gas in galaxies is consistent with observations of damped Lyα systems. The results suggest that the evolution of supermassive black holes, quasars, and starburst galaxies is closely linked to the hierarchical build-up of galaxies. The model uses semi-analytic techniques to simulate galaxy formation, including the merging history of dark matter haloes, cooling, star formation, and feedback cycles. The model shows that the evolution of the quasar luminosity function with redshift is well explained by the model, and that the observed evolution of the quasar population is consistent with the hierarchical build-up of galaxies. The model also fits the observed evolution of the star formation rate density, the total mass density in cold gas, and the luminosity function of galaxies. The model predicts that the number density of quasars declines with redshift due to the combination of three effects: a decrease in the merging rate, a decrease in the amount of cold gas available to fuel black holes, and an increase in the time-scale for gas accretion. The model also shows that the evolution of the quasar luminosity function is closely related to the evolution of the cold gas content in galaxies. The model is tested against observational data and shows good agreement with the observed evolution of the quasar population and the cold gas content in galaxies. The model also predicts that the luminosity of quasars declines with time after the merging event, and that the ratio of quasar luminosity to Eddington luminosity increases with redshift. The model is sensitive to assumptions about cooling, star formation, and accretion time-scales, and shows that the evolution of the quasar population is strongly influenced by these factors. The model also predicts that the host galaxies of quasars are