Variations in human genes are related to differences in the way people respond to drugs. When physicians prescribe a drug to a patient, they often are betting that the patient will have a "typical" response based on the doctor’s previous experience with the drug. It is often very difficult to anticipate when a drug will work particularly well or will cause significant side effects. Most people have had an experience taking a medication that did not work well or caused greater-than-expected side effects. There are a number of causes for variation in drug response across the population, but differences in genetics are an important factor. In this talk, we will review how genetic variation can affect drug response, and discuss a few examples of variation in commonly used medications. We will also touch upon the ethical issues that arise when using genetic information to make decisions about drug development and delivery.