The most important question about addiction asks; Is addiction a “disease” or a “choice”? This lecture outlines both arguments and provides evidence for why, in Addiction Medicine, doctors believe that addiction is in fact a disease. The most recent neurophysiologic research about addiction is presented, including the stress-induced hedonic dysregulation theory. The connection between this research and the baffling behaviors observed in addicts – behaviors such as craving, denial and persistent drug use despite negative consequences – is outlined.