Limitations in transformation capability can be a significant barrier in making advances in our understanding of gene function through the use of transgenics. To this end we have developed both tissue culture and non-tissue culture-based methodologies for the production of transgenic roots on wild-type shoots (composite plants). Composite plants are generated by inoculating wild-type shoots with Agrobacterium rhizogenes, which subsequently induces the formation of transgenic roots. The composite plant system allows for "in root" testing of transgenes in the context of a complete plant and can be analyzed in a variety of gene function analyses and plant-microbe interaction studies. In this chapter we provide a tissue culture-based composite plant generation system for Arabidopsis and a non-tissue culture based-method for producing composite plants on a variety of dicotyledonous plant species. Composite plants generated using these methods can be treated like "normal plants," planted in soil and grown in greenhouses or in growth chambers. These methods have been shown to work efficiently for many different species of plants including several that are recalcitrant to transformation.