By the early 1990's, it became possible to explain most of the spectral and color features of galaxies by linear combinations of SSP models. The model assumes a star formation rate, then takes an amount of stellar mass and converts it into a ZAMS population (zero-age main sequence). The population is then evolved using stellar evolution codes. At any particular timestep, the code is stopped, the stars counted by mass bin and a stellar spectral library is used to sum the integrated spectrum.

An example of one such model is shown below for a burst population (i.e. all the stars are made in a single burst). The model time (in Gyrs) is shown on the left hand side of each spectrum.

However, despite initial successes, there was a terrible non-uniqueness problem to SSP models. The age-metallicity degeneracy.