Marijuana has been used by humans for more than 2,500 years as a medicinal agent and social drug. Cannabinoids are the chemicals in marijuana that are responsible for its effects on the body. A long-standing interest of our laboratory is the study of the mechanisms by which the cannabinoids affect the function of the brain. Delta-9-tetrahydrocannabinol (THC) is the cannabinoid in marijuana that is responsible for its mood- and sensation-altering effects. THC targets two receptors: the CB1 receptor present on neurons and the CB2 receptor that is present primarily on immune cells. Although THC targets these receptors when a person is exposed to the drug from the outside, we know that at least two “endogenous” (or, from the self) molecules also target these receptors. These two molecules are named the endocannabinoids. Our research focuses on the cannabinoids, the receptors with which they interact and the role of the endocannabinoids in brain function.