This chapter describes the collision between these new genomic methodologies and the more conventional ways of discovering, developing and testing drugs, and point to ways in which both drug discovery and prescription are likely to be enhanced. The deployment of genomics within pharmaceutical research and development is markedly influencing both the efficiency and focus of the drug discovery and development process. Pharmaceutical discovery ultimately relies on establishing a set of pharmacological screens with which to profile novel chemical ‘leads’. The whole process of drug discovery and development, as we have seen, takes decades, and costs hundreds of millions of dollars, factors which have governed the rapid development of any technology which promises to optimize decision-making during the process. The mature approach involves ablation of individual gene products, in vitro or in vivo, and uses mainly post-transcriptional methods such as anti-sense oligonucleotide and intracellular antibody intervention to alter the expression of particular proteins and observe phenotypic sequelae.