Stellar population synthesis models at low and high redshift

The basic assumptions behind Population Synthesis and Spectral Evolution models are reviewed. The numerical problems encountered by the standard population synthesis technique when applied to models with truncated star formation rates are described. The Isochrone Synthesis algorithm is introduced as a means to circumvent these problems. A summary of results from the application of this algorithm to model galaxy spectra by Bruzual and Charlot (1993, 2000) follows. I present a comparison of these population synthesis model predictions with observed spectra and color magnitude diagrams for stellar systems of various ages and metallicities. It is argued that models built using different ingredients differ in the resulting values of some basic quantities (e.g. $M/L_V$), without need to invoking violations of physical principles. The range of allowed colors in the observer frame is explored for several galaxy redshifts.