Cannabis spp. are some of the most controversial medicinal plants in the world. They contain great amounts of biologically active secondary metabolites, including the typical phenolic compounds called cannabinoids. Because of their low toxicity and complex biological activities, cannabinoids can be useful in the therapy of various diseases, but adverse psychological effects (of Delta(9)-THC in particular) raise concerns. This review summarizes the current knowledge of selected active C. indica compounds and their therapeutic potential. We summarize the main compounds contained in cannabis, the mechanisms of their effects, and their potential therapeutic applications. Further, we mention some of the clinical tests used to evaluate the efficacy of cannabinoids in therapy.