This chapter provides readers with an introduction to some of the scientific, social and ethical dimensions of gene therapy. Gene addition therapy is the practice of administering a whole, healthy gene to a patient for the purpose of treating a genetic disease, though, in practice, this procedure varies widely according to disease type, body part treated and other factors. The chapter begins by explaining how genes can cause disease and how they can be used to treat disease, as well as discussing some of the technical challenges associated with this work. It then traces the history of the field, which began in the latter half of the 20th Century and saw the first patient treated in 1990. Scientific advances in gene therapy were closely tied to an evolving debate over the social and ethical merits and drawbacks of this work, which held great promise for treating disease but posed important questions about how far genetic interventions should go. Uncertainties about the field were compounded by controversial experiments, an American patients’ death, and the development of leukemia amongst several young European patients. The chapter reports on the state of gene therapy today: a few clinical trials have proven successful, but gene therapy is by no means a panacea for medicine more widely. New precise gene editing technologies are prompting renewed enthusiasm for and investment in the field, as well as raising further ethical questions to be explored in the future.