  The simplicity of hard spheres as a model system is deceptive. Although the
particles interact solely through volume exclusion, that nevertheless suffices
for a wealth of static and dynamical phenomena to emerge, making the model an
important target for achieving a comprehensive understanding of matter. In
addition, while real colloidal suspensions are typically governed by complex
interactions, Pusey and Van Megen [Nature 320 340--342 (1986)] demonstrated
that suitably tuned suspensions result in hard-sphere like behavior, thus
bringing a valuable experimental complement to the celebrated theoretical
model. Colloidal hard spheres are thus both a material in their own right and a
platform upon which phenomena exhibited by simple materials can be explored in
great detail. These various purposes enable a particular synergy between
experiment, theory and computer simulation. Here we review the extensive body
of work on hard spheres, ranging from their equilibrium properties such as
phase behavior, interfaces and confinement to some of the non--equilibrium
phenomena they exhibit such as sedimentation, glass formation and nucleation.
