  This is a review of the theoretical background, experimental techniques, and
phenomenology of what is called the "Glauber Model" in relativistic heavy ion
physics. This model is used to calculate "geometric" quantities, which are
typically expressed as impact parameter (b), number of participating nucleons
(N_part) and number of binary nucleon-nucleon collisions (N_coll). A brief
history of the original Glauber model is presented, with emphasis on its
development into the purely classical, geometric picture that is used for
present-day data analyses. Distinctions are made between the "optical limit"
and Monte Carlo approaches, which are often used interchangably but have some
essential differences in particular contexts. The methods used by the four RHIC
experiments are compared and contrasted, although the end results are
reassuringly similar for the various geometric observables. Finally, several
important RHIC measurements are highlighted that rely on geometric quantities,
estimated from Glauber calculations, to draw insight from experimental
observables. The status and future of Glauber modeling in the next generation
of heavy ion physics studies is briefly discussed.
