A number of research groups have used metabolic engineering approaches to alter volatile formation in a range of plant species. There are a number of metabolic pathways that can be targeted to engineer floral scent. Monoterpene pathway engineering was initially attempted in essential oil-containing plants, as these plants have specialized compartments for monoterpene production and are capable of producing large amounts of monoterpenes. Attempts to increase the production of sesquiterpenes by metabolic engineering have so far resulted in minor increases in the desired product, likely as a result of the limited precursor availability and tight regulation of the cytosolic mevalonate pathway. Modification of volatile formation in plants by metabolic engineering can also have effects on how the plant interacts with its environment. Scientific challenges will be the engineering of sesquiterpene formation and the fine-tuning of metabolic engineering by using specific promoters to target gene expression to where and when it is needed.