The terpenes are the largest class of plant natural products. This major class of compounds represents temendous chemical diversity of which only a realtively small fraction has so far been accessed and used by industry. The primary drivers of terpene diversification are terpenoid synthases and cytochromes P450, which synthesize and modify terpene scaffolds. Here, focusing on these two gene families, we investigate terpene synthesis and evolution across 17 sequenced plant genomes. Our analyses shed light on the roots of terpene biosynthesis and diversification in plants. They also reveal that different genomic mechanisms of pathway assembly pre-dominate in eudicots and monocots.