Like serotonin, dopamine is a neurotransmitter, a molecule that ferries messages from one neuron of the brain to another. Serotonin is associated with feelings of sadness and well-being, while dopamine is associated with pleasure and elation. Dopamine can be elevated by a hug, a kiss, or a word of praise and by the potent pleasures that come from drugs. Researchers do not suggest, however, that dopamine is the only chemical involved in drug addiction. Drugs modulate the activity of various brain chemicals, each of which intersects with many others. Nonetheless, the realization that dopamine may be a common end point of neural pathways represents a signal advance. The need to account for the effects of dopamine in preventing drug abuse is discussed, and biological mechanisms that contribute to dopamine's effects are examined. The dopamine cycle is described and illustrated, and the need for appropriate drug treatment is noted. Photographs and illustrations