Monoterpenes are compounds found in the essential oils extracted from many plants, including fruits, vegetables, spices and herbs. These compounds contribute to the flavor and aroma of plant from which they are extracted. Monoterpenes are acyclic, monocyclic, or bicyclic C30 compounds synthesized by monoterpene synthases using geranyl pyrophosphate (GPP) as substrate. GPP is also the precursor in the synthesis of farnesyl pyrophosphate (FPP) and geranyl-geranyl pyrophosphate (GGPP), two important compounds in cell metabolism of animals, plants and yeast. Monoterpene cyclases produce cyclic monoterpenes through a multistep mechanism involving a universal intermediate, a terpinyl cation which can be transformed to several compounds. Experimental studies, using animal cancer models, have demonstrated that some monoterpenes possess anticarcinogenic properties, acting at different cellular and molecular levels. From these discoveries it seems clear that monoterpenes could be considered as effective, nontoxic dietary antitumorigenic agents that hold promise as a novel class of anticancer drugs.