This chapter describes the degradative plasmids. Microorganisms play a major role in the degradation of the products and by-products of the activities of animals and plants and other microorganisms. The failure of microorganisms to rapidly recycle any of these compounds results in their accumulation in the environment and can lead to the phenomenon of environmental pollution. Recent advances in the genetic manipulation of microorganisms involving in vivo and in vitro genetic engineering techniques have provided microbiologists with an opportunity to study degradative pathways at the molecular level. An interesting feature of microorganisms that degrade a variety of aromatic and aliphatic compounds is that many of the genes encoding the degradative pathways are carried on plasmids. Degradative genes can be cloned in E. coli, followed by transfer of the cloned DNAs back into their original host to detect expression. Such experiments make use of small, broad host range cloning vectors and the well-defined genetic systems.