In the mid 1990s, the grid computing community promised the "compute power grid," a utility computing infrastructure for scientists and engineers. Since then, a variety of grids have been built worldwide, for academic purposes, specific application domains, and general production work. Understanding grid workloads is important for the design and tuning of future grid resource managers and applications, especially in the recent wake of commercial grids and clouds. This article presents an overview of the most important characteristics of grid workloads in the past seven years (2003-2010). Although grid user populations range from tens to hundreds of individuals, a few users dominate each grid's workload both in terms of consumed resources and the number of jobs submitted to the system. Real grid workloads include very few parallel jobs but many independent single-machine jobs (tasks) grouped into single "bags of tasks."