The common soil bacteria Agrobacterium tumefaciens and Agrobacterium rhizogenes are unique genetic pathogens capable of fundamentally redirecting plant metabolism in order to generate macroscopic tissue masses (crown galls and hairy roots, respectively) which support the growth of large populations of Agrobacteria. Central to pathogenesis is the horizontal transfer of a suite of oncogenes from the tumor-inducing (Ti) plasmids of A. tumefaciens and the root-inducing (Ri) plasmids of A. rhizogenes into the plant cell genome. These oncogenes alter the synthesis, perception and/or transport of phytohormones in planta, leading to the development of the crown gall and hairy root structures from single genetically transformed plant cells. Crown galls and hairy roots become effective sinks that divert plant resources to produce opine compounds that can only be metabolized by the infecting strain of Agrobacterium. The basic genetic and biochemical mechanisms underlying A. tumefaciens tumorigenesis were initially described over 20 years ago, with the characterization of the ipt, iaaM and iaaH oncogenes. However, the simplistic San Marcos, CA 92096, USA Ave, Davis, CA 95616, USA; Department of Biological Sciences, 524 Monica T. Britton, Matthew A. Escobar and Abhaya M. Dandekar view of crown gall development as solely a function of ipt-driven cytokinin synthesis and iaaM/iaaH-driven auxin synthesis has recently given way to a more nuanced understanding of the roles of secondary oncogenes in modulating hormone perception and the complex hormone activation cascade in crown galls involving ethylene, abscisic acid and jasmonic acid. The biochemistry and functional significance of specific oncogenes in A. rhizogenesmediated hairy root development is less well understood, but recent work has substantially increased our understanding of the A. rhizogenes oncogenes, especially the rol genes. Expression of the rolA, B and C oncogenes in planta induces a subtle interaction with endogenous plant signal transduction pathways and transcription factors, affecting the local concentrations of several classes of plant hormones. These interactions lead to de novo meristem formation in transformed cells, with subsequent differentiation depending on the local hormone balance. This process most often results in the induction of highly branched non-geotropic adventitious roots, the “hairy root” phenotype. Further dissection of the molecular mechanisms underlying Agrobacterium pathogenesis should continue to yield broader insights into the understanding of endogenous hormone signaling pathways and tissue differentiation in plants.