Increasingly, complex scientific applications are struc- tured in terms of workflows. These applications are usu- ally computationally and/or data intensive and thus are well suited for execution in grid environments. Distributed, ge- ographically spread computing and storage resources are made available to scientists belonging to Virtual Organiza- tions sharing resources across multiple administrative do- mains through established service-level agreements. Grids provide an unprecedented opportunity for distributed work- flow execution; indeed, many applications are well beyond the capabilities of a single computer, and partitioning the overall computation on different components whose execu- tion may benefit from runs on different architectures could provide better performances. In this paper we describe the design and implementation of the Grid Resource Bro- ker (GRB) Workflow engine.