As more businesses engage in globalization, inter-organizational collaborative computing grows in importance. Since we cannot expect homogeneous computing environments in participating organizations, heterogeneity and Internet-based technology are prevalent in inter-organizational collaborative computing environments. One technology that provides solutions for data sharing and work coordination at the global level is inter-organizational workflow. In this paper, we investigate the access control requirements for inter-organizational workflow. We then present access control solutions for inter-organizational workflow based on our implementation. Many of the requirements and solutions in this paper address the scalability of existing security solutions, the separation of inter-organizational workflow security from concrete organization level security enforcement, and the enforcement of fine-grained access control for inter-organizational workflow.