Cocaine produces its psychoactive and addictive effects primarily by acting on the brain's limbic system, a set of interconnected regions that regulate pleasure and motivation. An initial, short-term effect -- a buildup of the neurochemical dopamine -- gives rise to euphoria and a desire to take the drug again. Researchers are seeking to understand how cocaine's many longer term effects produce addiction's persistent cravings and risk of relapse. In the author's laboratory, work has focused on buildup of the genetic transcription factor DeltaFosB. Levels of DeltaFosB in the limbic system correlate with addiction-like behaviors in mice and may precipitate very long-lasting changes to nerve cell structure. Further pursuit of this and similar leads are first steps toward a complete understanding of the transition from cocaine abuse to addiction -- and, ultimately, more effective treatments for those who are addicted.