  Today's sensistive, high resolution Chandra X-ray observations allow the
study of many populations of X-ray sources. The traditional astronomical tools
of photometric diagrams and luminosity functions are now applied to these
populations, and provide the means for classifying the X-ray sources and
probing their evolution. While overall stellar mass drives the amount of X-ray
binaries in old stellar population, the amount of sources in star-forming
galaxies is related to the star formation rate. Shart-lived, luminous, high
mass binaries (HNXBs) dominate these young populations.
