Arachis hypogea (peanut, groundnut), an annual oil seed belonging to the Leguminosae family and the Papillionacea subfamily, is a legume native to South America but now grown in diverse environments in six continents between latitudes 40 degrees N and 40 degrees S. Arachis hypogea can grow in a wide range of climatic conditions. The low yields of this crop are mainly attributed to unreliable rainfall patterns with frequent droughts, lack of high-yielding adapted cultivars, damage by diseases and pests, poor agronomic practices, and limited use of inputs. Genetic engineering approaches have been shown to be comparatively fast, leading to better isolation and cloning of desired traits for combating the various biotic and abiotic stresses. This chapter describes an Agrobacterium-mediated transformation protocol in peanut using the cotyledon system. The system described here is potentially applicable to a vast range of genotypes with a high transformation frequency of >70% based on the preliminary molecular data, indicating the production of a large number of independently transformed transgenic plants. The method reported here provides opportunities for crop improvement of this important legume crop via genetic transformation.