Virulent strains of Agrobacterium tumefaciens and Agrobacterium rhizogenes induce non-self limiting — neoplastic — growths on susceptible plants, generally in dicotyledonous species from the Angiosperms. In most cases, A. tumefaciens induces unorganized “crown gall” tumors (so named because the growths were often observed at the crown of the plant), though certain isolates can induce teratomatous tumors that exhibit a choatic array of plant structures (Figure 1). A. rhizogenes infection results in the continuous proliferation of “hairy roots” from the infection site (Figure 2). The demonstration that A. tumefaciens (then called Bacterium tumefaciens) is the causal agent of crown gall tumors was first presented by Smith and Townsend (1907) who showed that this bacterium could fulfill Koch’s postulates. Later, Riker et al. (1930) showed A. rhizogenes was, similarly, the causal agent of the hairy root disease.