I will illustrate the content of a book by Alvio Renzini and myself
recently published by WILEY-VCH. The book is aimed at describing how
stellar populations work, specifically with respect to those tools
commonly used to interpret the properties of galaxies in terms of
their star formation history.  The fundamentals on which these tools
are constructed, as well as their limitations, are disclosed, so as to
illustrate how much can be learned on galaxy formation and evolution
from the analysis of the light from their stellar populations.

After a summary of the content of the book, I will illustrate in some
detail two chapters pertaining to the study of resolved stars in
galaxies. In particular, I will discuss the errors which affect
stellar photometry in crowded fields, and elucidate the internal
workings of the synthetic color-magnitude diagram method, which is
widely used to derive the star formation history.