Agrobacterium has evolved the capacity of delivering to and integrate into the plant genome bacterial oncogenes the expression of which deeply affects normal plant development. Unlike the oncogenes transferred from A. tumefaciens, the oncogenes transferred from A. rhizogenes (T-DNA genes) seem to have a very complex mechanism of action and are finely regulated by plant regulatory factors. This review focuses on the rol oncogenes, because of the central role they play in root induction. A critical survey of other relavant T-DNA genes and of the cellular T-DNA homologues found in untransformed plant genomes, is also presented.