The substance related disorders encompass 10 separate classes of drugs: alcohol, caffeine, cannabis, hallucinogens (with separate categories for phencyclidine [or similarly acting arylcyclohexamines and other hallucinogens), inhalants, opioids, sedatives, hypnotics, and anxiolytics, stimulants (amphetamine-type substances, cocaine, and other stimulants), tobacco, and other (or unknown) substances. These 10 classes are not fully distinct. All drugs that are taken in excess have in common direct activation of the brain reward system, which is involved in the reinforcement of behaviors and the production of memories. They produce such an intense activation of the reward system that normal activities may be neglected. Instead of achieving reward system activation through adaptive behaviors, drugs of abuse directly activate the reward pathways. The pharmacological mechanisms by which each class of drugs produces reward are different, but the drugs typically activate the system and produce feelings of pleasure, often referred to as a ”high.” Furthermore, individuals with lower levels of self-control, which may reflect impairments of brain inhibitory mechanisms, may be particularly predisposed to develop substance use disorders, suggesting that the roots of substance use disorders for some persons can be seen in behaviors long before the onset of actual substance use itself.