Cannabinoids, consisting of alkylresorcinol and monoterpene groups, are the unique secondary metabolites that are found only in Cannabis sativa. Tetrahydrocannabinol (THC), cannabidiol (CBD) and cannabichromene (CBC) are well known cannabinoids and their pharmacological properties have been extensively studied. Recently, biosynthetic pathways of these cannabinoids have been successfully established. Several biosynthetic enzymes including geranylpyrophosphate:olivetolate geranyltransferase, tetrahydrocannabinolic acid (THCA) synthase, cannabidiolic acid (CBDA) synthase and cannabichromenic acid (CBCA) synthase have been purified from young rapidly expanding leaves of C. sativa. In addition, molecular cloning, characterization and localization of THCA synthase have been recently reported. THCA and cannabigerolic acid (CBGA), its substrate, were shown to be apoptosis-inducing agents that might play a role in plant defense. Transgenic tobacco hairy roots expressing THCA synthase can produce THCA upon feeding of CBGA. These results open the way for biotechnological production of cannabinoids in the future.