Psychiatric disorders such as drug addiction represent one of the biggest challenges to society. This article reviews clinical and basic science research to illustrate how developments in research methodology have enabled neuroscientists to understand more about the brain mechanisms involved in addiction biology. Treating addiction represents a major challenge and current therapies are limited. Although short-term reductions in drug-taking are achieved, a significant proportion of patients relapse after several months; understanding why this occurs and finding treatments to prevent relapse to drug-taking are major challenges for the 21st century. (Contains 2 figures and 2 tables.)