THE formation of two-dimensional patterns in biological, chemical and physical systems is often described by the nonlinear interaction of plane waves1. An alternative approach views patterns as ensembles of interacting localized objects, analogous to the assembly of crystals from atoms. For macroscopic pattern-forming systems, one objection to the latter approach is that no 'atoms' exist; however spatially localized excitations can play an analogous role. One-dimensional localized states are observed in many systems—for example, solitary waves in water2–4 and optical fibres5—and can organize into simple patterns6,7. But few examples of two-dimensional localized states are known, and these tend to be unstable and/or do not show simple pattern-forming interactions8–11. Here we report the observation of stable, two-dimensional localized excitations zin a vibrating layer of sand. These excitations, which we term 'oscillons', have a propensity to assemble into 'molecular' and 'crystalline' structures. Our experimental results, together with the observation of similar localized excitations in model differential equations12–14, indicate a crucial, cooperative role for hysteresis and dissipation in the formation of oscillons, and suggest that similar behaviour may occur in continuous media.