In an optical lattice the interaction of an atomic gas with a spatially modulated light field is exploited to damp the atomic motion and then to trap the slowly moving atoms on a regular array of points associated with particular local states of the light field. The result of this process is the formation, from the initially cold but disordered gas, of a structure reminiscent of those found in solid-state crystalline materials. The study of these novel, optically-bound media necessitates the introduction to the description of the atom-light interaction of concepts more usually encountered in solid-state physics and promises to shed new light on the nature of atom-light interactions. Additionally, it provides a new means for the precise and highly parallel manipulation of atoms and a novel test-bed for some fundamental problems in physics.