— This paper investigates Grid computing from the point of view three basic computing platforms. The platform consists of virtual compute resources, a programming environment allowing for the development of grid applications, and a grid operating system to execute user programs and to make solving complex user problems easier. Three platforms are discussed: compute Grid, metacompute Grid and Intergrid. Service protocol-oriented architectures are contrasted with service object-oriented architectures, then the SORCER metacompute Grid based on the service object-oriented paradigm is presented. Finally, we explain how SORCER, with its core services and federated file system, can be used as a traditional compute Grid and an Intergrid—a hybrid of compute and metacompute Grids.