This chapter looks at terpenes, which form the largest class of molecules found in essential oils. It is of interest to aromatherapists how terpenoid molecules behave when they come in contact with the substances found in human bodies, such as skin oils, cell membranes and the variety of substances in blood and other body fluids. Most molecules containing electronegative atoms are polar molecules. Non-polar molecules are molecules in which there are no strongly electronegative atoms, or if there are, their electron-drawing powers cancel each other out. The property of solubility is one of the most important for aroma-therapists to understand, as it applies not only to the use of different solvents in blending, but also to the mechanism of epidermal penetration of the essential oils, and eventual transport of essential oils in the blood. Ethanol is a solvent often used to dissolve essential oils in perfumery. Most monoterpenoid constituents are moderately soluble in ethanol.