The mechanisms of action, biosynthesis, inactivation and structure – activity relationships of endocannabinoids are reviewed. The discovery of endogenous cannabinoid system was an important step in understanding the psychotropic action of cannabis. The activation of cannabinoid receptor CB₁ is responsible for psychotropic effects of cannabinoids; however, some effects of these drugs may be also mediated by nonreceptor mechanisms, e.g., due to changes in the lipid part of biological membranes. The role of the lipid bilayer in the cannabinoid system activity is indicated by the fact that membrane lipids are the source and pantry of endocannabinoids and main endocannabinoids are derived from arachidonic acid, which is an important part of membrane lipids. The structure – activity relationship of cannabinoids (plant, endogenous, synthetic) and other substances affecting the cannabinoid system is intensively studied. The purpose of the studies is the discovery of drugs with therapeutic actions similar to cannabis, but without its psychoactivity.