The three dimensional structure of the atoms in a molecule, and how it appears in space. Because two molecules that are identical in composition may have different stereochemistries, the placement of functional groups and side chains can affect the activity of a molecule, and in terms of therapeutics, can mean the difference between an active and inactive version of a molecule or a drug. Stereochemistry is the basis of circular dichroism. See also Circular Dichroism, Structural Biology.