Terpenes (/ˈtɜːrpiːnz/ TUR-peenz) are a large and diverse class of natural products with the general formula (C₅H₈)_n, where n ≥ 2. They serve as crucial biosynthetic building blocks in many organisms, particularly plants. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predominantly by plants, particularly conifers. In plants, terpenes and terpenoids are important mediators of ecological interactions, while some insects use some terpenes as a form of defense. Other functions of terpenoids include cell growth modulation and plant elongation, light harvesting and photoprotection, and membrane permeability and fluidity control.

Terpenes are classified by the number of carbons: monoterpenes (C₁₀), sesquiterpenes (C₁₅), diterpenes (C₂₀), as examples. The terpene alpha-pinene is a major component of turpentine, a common solvent.

One terpene that has major applications is natural rubber (i.e., polyisoprene). The possibility that other terpenes could be used as precursors to produce synthetic polymers has been investigated. Many terpenes have been shown to have pharmacological effects. Terpenes are also components of some traditional medicines, such as aromatherapy, and as active ingredients of pesticides in agriculture.