Increasingly complex population synthesis models have been developed during the last few years. Among the different authors working in the field, these groups have been particularly active: Arimoto & Yoshii (1987), Guiderdoni & Rocca-Volmerange (1987), Buzzoni (1989), Fritze-v. Alvensleben & Gerhard (1994), Bressan, Chiosi, & Fagotto (1994), Bruzual & Chariot (1993, 1995). The basic astrophysical ingredients used in these models are the stellar evolutionary tracks and the stellar spectral libraries (either empirical or theoretical). The computational algorithms are different in each set of models, but, one way or the other, these models depend on the same adjustable parametric functions: (1) the stellar initial mass function (IMF); (2) the star formation rate (SFR); and (3) the rate of chemical enrichment (in some models Z = Z
⊙ = constant). Most of the recent codes use the isochrone synthesis algorithm, which allows to compute the evolution of a simple stellar population (SSP), and from it, by a convolution integral, the properties of more complex composite stellar populations (CSP).