A comprehensive investigation is presented on the factors governing the photoluminescence (PL) quantum yields (QYs) and size dispersion of colloidal CdSe nanocrystals. The temporal evolution of the ensemble PL properties (absorption and luminescence spectra, QYs and lifetimes) during growth at different temperatures (170-310 °C) and different Cd:Se ratios was followed for several hours (2-6 h). The QY values increase during the growth to a maximum and, after a variable time interval (from minutes to hours, depending on the growth temperature), decrease gradually. Low QYs are due to poor passivation, surface disorder, and surface degradation, which arise at different stages of the growth. High QYs can be achieved and maintained only under an ideal combination of growth temperature, solvent composition, and Cd:Se ratio, which leads to an optimum surface. The overgrowth of a fresh surface layer restores high QYs to CdSe nanocrystals with decreased efficiencies because of surface degradation. The insight gained in this investigation enabled us to achieve a high degree of reproducibility and control over the emission color (green to red), bandwidth (90 meV), lifetimes (30 ns), and quantum yields (50-85%) of colloidal CdSe nanocrystals without any postpreparative treatment.