Substance abuse and addiction are complex phenomena that defy simple explanation or description. A tangled interaction of factors contributes to an individual's experimentation with, use, and perhaps subsequent abuse of drugs. Regardless of the mix of contributing factors, the actions and effects exerted by drugs of abuse underlie all substance abuse and addiction. In order to understand substance abuse and addiction it is first necessary to understand how drugs work in the brain, why certain drugs have the potential for being abused, and what, if any, biological differences exist between individuals in their susceptibility to abuse drugs. This background paper describes biological contributing factors to substance abuse and addiction.