International collaborations are now based on shared collections that span multiple institutions' administrative domains. The collections are managed by data grids, which implement global persistent name spaces for managing shared data. They also provide trust virtualizationÂ—the ability to assert authenticity and authorization independently of the underlying storage resources. We will examine how trust virtualization is managed and the implications for security requirements. Examples from multiple international projects that use federation of data grids to increase local control of resources will illustrate various challenges. We will also look at pragmatic issues related to data integrity for petascale collections.