  Star clusters - open and globulars - experience dynamical evolution on time
scales shorter than their age. Consequently, open and globular clusters provide
us with unique dynamical laboratories for learning about two-body relaxation,
mass segregation from equipartition of energy, and core collapse. We review
briefly, in the framework of star clusters, some elements related to the
theoretical expectation of mass segregation, the results from N-body and other
computer simulations, as well as the now substantial clear observational
evidence.
