Diterpenoids are chemical compounds containing 20 carbon atoms and belong to the terpenoid class. They derive from geranylgeraniol, a C20 precursor, have a C20H32 basic structure, and are composed of four isoprene units. These features make diterpenoids different from simple terpenes, which possess only 10 carbon atoms. A diterpenoid molecule may also include alcohol, phenol, aldehyde, cheton, or acidic functional groups. These compounds are highly lipophilic, odorless, and may possess strong flavours. They are found mainly in fungi and in resins of higher-order plants, as typical products of plant metabolism. This book examines the types, functions and provides new research on diterpenoids.