NIDA's mission is to lead the Nation in bringing the power of science to bear on drug abuse and addiction. Recent scientific advances have revolutionized our understanding of drug abuse and addiction, which we now recognize as a chronic relapsing brain disease expressed in the form of compulsive behaviors. This understanding has improved our ability to both prevent and treat addiction.

http://www.nida.nih.gov/about/welcome/mission/NIDA_Movie1.html