Many researchers use the flowering plant Arabidopsis thaliana to study gene function and basic plant biology. This easy-to-grow, small plant is ideal for genetic studies as it has a relatively simple genome compared to crop plants and its genetic material has been recently sequenced. Another very useful feature of Arabidopsis is that it is extremely simple to transform genetically. The ability to insert genes of interest stably into a given plant is essential to understand and verify gene function. Transformation is also a means of introducing specific traits that are difficult or impossible to introduce by conventional breeding techniques. This chapter provides detailed explanations on the floral dip protocol, a simple method to transform Arabidopsis by inoculating immature flowers with Agrobacterium tumefaciens.