Rapid development of plantation forestry is necessary to meet an increasing demand of fiber and wood for the forest products industry. In addition to delivering high productivity and high quality of cellulose and lignin formation, trees for plantation forestry must resist increasing pressure from insect pests and pathogens. Unlike in agriculture, as practiced in much of the 20th century, forestry cannot rely on massive application of pesticides for crop protection, because forests, including plantation forests, are complex ecosystems providing essential habitats for soil living and above ground organisms. At the current stage of genetic tree improvement, there are unique opportunities to retain or recapture much of the genetic material that determines natural defense and resistance for plantation forestry. Terpenoids and terpenoid synthase (TPS) genes are major components of defense and resistance in conifers and appear to be important for defense biology of poplars as well. Terpenoid defense in forest trees present multigenic traits that are highly variable within species and natural populations. Much of the chemical diversity of terpenoids in trees is determined by variable constitutive and inducible expression of members of large TPS gene families and by multi-product reaction mechanisms of TPS enzymes. Detailed knowledge of the organization and evolution of TPS gene families, TPS gene expression and TPS enzyme biochemistry is critical to maintain natural genetic and chemical diversity of terpenoids in trees selected and improved for plantation forestry. TPS genes can be targeted for development of markers for pest resistance and also as genetic markers for formation of terpenoid extractives that affect mills and toxicity of efflux water in the pulp and paper industries. In addition, the biochemical machinery of resin terpenoid formation and resin secretion in conifers can conceivably be harnessed for biotechnological production of high value compounds in renewable plantation forest resources. Conifers and poplars also emit massive amounts of reactive hemiterpenes and monoterpenes into the atmosphere. Another system of plantation forestry, Eucalyptus (not discussed in this chapter due to lack of published studies of TPS genes in this system) also emits monoterpenes in large quantitities and is protected against many herbivores by terpenoids.