Agrobacterium tumefaciens is the causal agent of the plant disease called crowngall, and it’s able to infect more than 90 families of dicotyledonous plants. It is an α-Proteobacterium and belongs to the Rhizobiaceae family. A. tumefaciens is a complex of different species grouped in 10 genomovars (G1 to G8, and G13). A. tumefaciens C58 belongs to the G8 group. Its genome consists in 4 replicons: 1 chromosome circular, 1 chromosome linear and 2 dispensable plasmids: pAt (for A. tumefaciens) and pTi (for Tumor inducing), which is required for virulence. To explore new aspects of the A. tumefaciens lifestyle, and in particular the interaction between the bacteria and its plant host, two different approaches have been used to identify, characterize and analyze genes that could play a role in the adaptation of the bacteria to tumor lifestyle. An evolution experiment by serial passages of three different strains of thepathogen on the host plant Solanum lycopersicum has been carried out to clarify the evolutionary dynamics of the genome during the course of infection. In parallel, a study of different transcriptomes (in planta and in vitro) was performed and studied to elucidate bacterial candidate genes involved in the interaction of the bacteria with the plant and various compounds produced in the tumor. This work attempts to give a more general view of the process of adaptation of the bacteria to the ecological niche that is the tumor.