**2. Monoterpenes**

Monoterpenes consist of 10 carbon atoms with two isoprene units and molecular formula C10H16. These are naturally present in the essential and fixed oils of plants and related sources. Monoterpenes are structurally divided into the acyclic, monocyclic, and bicyclic type of


**Table 1.** Source and biological activities of some monoterpenes.

**Figure 2.** Structure of monoterpene.

compound. The compounds belong to this class usually have strong aroma and odor and are used in many pharmaceutical companies. Mixture of different monoterpene-based oils is used as fragrances for making perfumes and in other cosmetics. Most of the monoterpenes are active biologically with strong antibacterial activities. Several studies have shown *in vitro* and *in vivo* antitumor activity of many essential oils obtained from plants. The antitumor activity of essential oils of many species has been related to the presence of monoterpenes in their composition [1]. Herein, we are discussing some of the recently published active monoterpenes (**Table 1**, **Figure 2**).
