Sub-micrometre charged latex spheres can be suspended In water to form regular arrays known as colloidal crystals. In contrast to most conventional solids, colloidal crystals can be forced Into metastable superheated states, The structure and dynamics of these metastable crystals show evidence for strong, long-range attractions between the similarly charged spheres. Such attractive interactions are inconsistent with the accepted theory of colloidal interactions, and might influence the properties of many natural and Industrial suspensions.