Plant transformation has emerged as a key technique for investigations on plant gene function as well as crop development. Since the early 1900s, Agrobacterium tumefaciens and several related Agrobacterium species have been identified as plant diseases. Nevertheless, the ability of Agrobacterium to transfer DNA to plant cells has just been used for plant genetic engineering in the last 20 years. Initially, Agrobacterium was only used to transform dicot plants, but as the plant transformation protocol changed over the years, it was also used to transform monocots such as wheat, rice, and maize. The process of Agrobacterium-mediated plant transformation is quite complicated and developed, involving the genetic determinants of the host plant cell as well as the bacterium. The Agrobacterium-mediated gene transfer method acts as nature genetic engineer to understand basic plant metabolic processes as well as to exploit the system for biotechnological applications.