Grid computing offers significant enhancements to our capabilities for computation, information processing and collaboration, and has exciting ambitions in many fields of endeavour. In this paper we argue that the full richness of the Grid vision, with its application in e-Science, e-Research or e-Business, requires the ?Semantic Grid?. The Semantic Grid is an extension of the current Grid in which information and services are given well-defined meaning, better enabling computers and people to work in cooperation. To this end, we outline the requirements of the Semantic Grid, discuss the state of the art in achieving them, and identify the key research challenges in realising this vision.