The essential oils of Thymus, Melaleuca, and Eucalyptus species are natural resources for a wide range of pharmaceutical valued monoterpenes. The present study is about the stereospecific and regiospecific mechanisms of the enzymes involved in the formation of oxygenated, cyclic monoterpenes in these species. Therefore, genes encoding for the respective enzymes were isolated, heterologously expressed, and characterized. From Thymus vulgaris two sabinene hydrate synthases were isolated which formed the opposite enantiomers of their chiral monoterpene products. Only one amino acid residue in the active site of both enzymes determined most of the stereospecificity. For the formation of thymol and carvacrol, a pathway from γ-terpinene catalyzed by the combined activity of a cytochrome P450 enzyme and a dehydrogenase was postulated. From Melaleuca and Eucalyptus species, putative monoterpene synthases and cytochrome P450 enzymes could be isolated.