Limonene is a monoterpene with a clear colourless liquid at room temperature, a naturally occurring chemical which is the major component in oil of oranges. Limonene is widely used as a flavor and fragrance and is listed to be generally recognized as safe in food by the Food and Drug Administration (21 CFR 182. 60 in the Code of Federal Regulations, U.S.A.). Limonene is a botanical (plant-derived) solvent of low toxicity. Mild skin irritation may occur from exposure to limonene and oxidation products of limonene may produce dermal sensitization, and may have irritative and bronchoconstrictive airway effects; however, data are scant and more studies are required. Limonene has been shown to cause a male rat-specific kidney toxicity referred to as hyaline droplet nephropathy. Furthermore, chronic exposure to limonene causes a significant incidence of renal tubular tumors exclusively in male rats. Limonene is one of the active components of dietary phytochemicals that appears to be protective against cancer. Limonene, (+)- is an oral dietary supplement containing a natural cyclic monoterpene and major component of the oil extracted from citrus peels with potential chemopreventive and antitumor activities. Although the mechanism of action has yet to be fully elucidated, limonene and its metabolites perillic acid, dihydroperillic acid, uroterpenol and limonene 1,2-diol may inhibit tumor growth through inhibition of p21-dependent signaling and may induce apoptosis via the induction of the transforming growth factor beta-signaling pathway. In addition, they inhibit post-translational modification of signal transduction proteins, resulting in G1 cell cycle arrest as well as differential expression of cell cycle- and apoptosis-related genes.