This review describes how recombinant DNA technology has been used to improve Bacillus thuringiensis (Bt) products and overcome a number of the problems associated with Bt-based insect control measures. It will discuss how the knowledge of the genetics of Bt and of its insecticidal toxin genes, the understanding of their regulation and the development of cloning vectors has made possible the continuing improvement of first generation products. Several examples describing how biotechnology has been used to increase the production of insecticidal proteins in Bt, their persistence in the field by protecting them against UV degradation or to construct non-viable genetically modified strains, will be presented.