  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.
