Genetic variation in the human genome occurs predominantly as single nucleotide polymorphisms (SNPs). Our DNA may contain as many as ten million SNPs, of which three million or more are likely to differ between any two unrelated individuals. These three million genetic differences make a significant contribution to the observed variation in complex human phenotypes, such as disease susceptibility and our responses to drugs or environmental chemicals. Largescale association studies taking place throughout the drug development process can help to identify such differences and tailor drugs and dose regimens to particular genotype classes. The need for such large-scale studies has driven the development of high-throughout SNP discovery and typing technologies, which are the subject of this review.