Research on crown gall tumors began as a classic study in plant pathology aimed at identifying the cause of a devastating plant disease with unusual symptoms. Studies on Agrobacterium by investigators in laboratories around the world have revealed new biological phenomena and practical applications far beyond what Smith and Townsend could ever have imagined. On the one hand, Agrobacterium causes serious problems for growers of grapes, stone fruits, and ornamental plants. On the other hand, it has provided an unusually exciting and rewarding experimental system for those of us fortunate to have entered into its study. The study of Agrobacterium-plant interactions has led to many unanticipated pleasant surprises, many of which were first observed in this system but now have been found to be common in the biological world.