Microbial activity in the rhizosphere can have positive and negative effects on plants. Some of the beneficial processes act by minimizing the negative effects or by modifying the cropping environment to enhance productivity. Processes that are considered here include the provision of nitrogen, phosphorus and iron to the plant, the biocontrol of diseases and deleterious organisms and the stabilization of soil structures. The use of genetic engineering techniques in studying these processes and generating novel strains which may enhance them is discussed, along with the possible consequences of the release of organisms into the environment.