Conifers are gymnosperms that form a large group of woody plants that have worldwide distribution with a long evolutionary history. A large number of conifers produce oleoresins or resins that are stored in resin ducts. These oleoresins and resins have a wide range of applications in pharmacy, fragrance, and flavoring industry and further industrial products. Diterpenes are the major compounds of the non-volatile fraction of such resinous products. In terpene metabolism diterpenes are formed from four isoprene units giving compounds with 20 carbon atoms that are arranged in a ring system. The major diterpene scaffolds are abietanes, labdanes, pimaranes, kauranes, and cembranes. Further rare diterpene types occur in certain species. Neutral diterpenes occur as unsaturated diterpene hydrocarbons or alcohols, aldehydes, and ketones. Acidic compounds form the resin acid which constitute a major part of the non-volatile fraction of oleoresins. The present chapter gives an overview of common diterpenes in conifers as well as some examples of rare structures. Diterpenes are part of the defense systems of conifers against pathogens and herbivores due to antimicrobial or antifeedant properties. In damaged tissues diterpene resins are sealing the wound. Numerous diterpenes display biological activities and are of interest for pharmacy and other uses. Exploring the mechanisms of action and developing new applications are promising research fields.