Vibrational circular dichroism (VCD) is a form of chiroptical spectroscopy for which chiral molecules have non-zero spectra with intensities that are identical for mirror-image pairs (enantiomers) of chiral molecules but with signs that are opposite with respect to zero intensity. For a chiral molecular substance, two important properties are 1) absolute configuration (AC), which indicates which of two mirror-image forms of the molecule is present; and 2) enantiomeric excess (EE, also called enantiomeric purity), the fractional excess of one enantiomer of a chiral molecule over its mirror-image enantiomer. The principal chapter in USP–NF that addresses molecular chirality is Optical Rotation 〈781〉. Optical rotation (OR) and electronic circular dichroism (ECD) are the traditional forms of chiroptical spectroscopy, whereas VCD is a newer form of chiroptical spectroscopy that contains more molecular structure information and recently has been adopted more broadly for use in the pharmaceutical industry.