G. Bruzual and S. Charlot present a new model for computing the spectral evolution of stellar populations over a wide range of metallicities and ages, from 1 × 10^5 to 2 × 10^10 years. The model is based on a newly available library of observed stellar spectra and incorporates recent progress in stellar evolution theory, including an observationally motivated prescription for thermally pulsing stars on the asymptotic giant branch (AGB). The model is designed to reproduce the observed optical and near-infrared color-magnitude diagrams of Galactic star clusters and to constrain physical parameters such as star formation history, metallicity, and dust content of galaxies. The authors compare their model with observed galaxy spectra from the Sloan Digital Sky Survey (SDSS) and discuss the limitations and advantages of their approach. They also explore the influence of adjustable parameters on the photometric evolution of simple stellar populations (SSPs) and compare their results with previous work. The model is the first to enable accurate studies of absorption-line strengths in galaxies containing stars over the full range of ages, using high-quality spectra from the SDSS.G. Bruzual and S. Charlot present a new model for computing the spectral evolution of stellar populations over a wide range of metallicities and ages, from 1 × 10^5 to 2 × 10^10 years. The model is based on a newly available library of observed stellar spectra and incorporates recent progress in stellar evolution theory, including an observationally motivated prescription for thermally pulsing stars on the asymptotic giant branch (AGB). The model is designed to reproduce the observed optical and near-infrared color-magnitude diagrams of Galactic star clusters and to constrain physical parameters such as star formation history, metallicity, and dust content of galaxies. The authors compare their model with observed galaxy spectra from the Sloan Digital Sky Survey (SDSS) and discuss the limitations and advantages of their approach. They also explore the influence of adjustable parameters on the photometric evolution of simple stellar populations (SSPs) and compare their results with previous work. The model is the first to enable accurate studies of absorption-line strengths in galaxies containing stars over the full range of ages, using high-quality spectra from the SDSS.