Grid computing is an emerging form of distributed computing, distinguished from traditional forms by its focus on large-scale, multi-organizational resource sharing and innovative applications. To harness shared resources across multiple organizations, and at the same time to deal with the challenges of grid resource management and of deploying grid applications, we have developed KOALA, a Grid Resource Management System. KOALA supports co-allocation, i.e., the allocation of both processors and data to single applications in multiple sites and the simultaneous access to these resources by the applications. By supporting co-allocation, KOALA addresses new challenges presented to resource management in grids by co-allocation of allocating resources in multiple sites, guaranteeing the simultaneous availability of the co-allocated resources, and managing sets of highly dynamic grid resources. KOALA also has mechanisms that simplify the deployment of different application types on the grid. Without these mechanisms, deployment of different application types on grids is difficult because of the characteristics of the grid applications and of the grid infrastructure. KOALA has been deployed on the Distributed ASCI Supercomputer (DAS), which is an experimental computer testbed in the Netherlands that is exclusively used for research on parallel, distributed, and grid computing. KOALA has proven to be working reliably on the DAS testbed with over 500,000 jobs already submitted to it.