The formation of hairy roots occurs after infecting the plants by Agrobacterium rhizogenes. These transformed roots are characterized by a rapid growth in the hormone free medium. They are ageotropic in growth and have extensive lateral branching. The pathological rhizogenicity arises from the stable insertion of a region of the Ri (root-inducing) plasmid into the plant nuclear genome. This plasmid can also be engineered to contain foreign genes, which can also be stably inserted into the host genome. This chapter gives an overview of hairy root formation and the role of different genes involved in the process of boosting the production of secondary metabolites.