In collaboration with the Advanced Research Consortium (Texas A&M), three scholars from the Universities of Michigan and Illinois, Urbana Champagne are assembling a thematic collection on the environmental history of the Great Lakes Basin. The Great Lakes Aggregator pushes the envelope on the theory and practice of thematic collections in the humanities. The talk will establish the scholarly context for digital aggregation and then summarize some of the conceptual and technical challenges of thematic aggregation, including the choice of collections, the transformation and normalization of metadata, and the development of user guides for previously dispersed digital collections. The presentation will conclude with some insights on the future of digital collection aggregation in support of digital scholarship in the humanities. The project is supported by a grant from the Humanities Without Walls initiative.