The study of marijuana cannabinoid biology has led to many important discoveries in neuroscience and immunology. These studies have uncovered a new physiological system, the endocannabinoid system, which operates in the regulation of not only brain function but also the regulation of the immune system. Studies examining the effect of cannabinoid-based drugs on immunity have shown that many cellular and cytokine mechanisms are suppressed by these agents leading to the hypothesis that these drugs may be of value in the management of chronic inflammatory diseases. In this report, we review current information on cannabinoid ligand and receptor biology, mechanisms involved in immune suppression by cannabinoids with emphasis on antigen-presenting cells, and preclinical and clinical models analyzing the therapeutic potential of cannabinoid-based drugs.