We will now explore the principal methods for the description and calculation of the electronic distribution in molecules. The importance of having knowledge of a biological molecule’s electron distribution cannot be overstated. The electronic distribution in a molecule is responsible for its chemical properties. As we will see, the formation of chemical bonds with the consequent bond strengths, lengths, force constants, and angles defines the shape and reactivity of molecules. The polarity of bonds determines charge distribution and give rise to the dipole moment of a molecule. The interaction of the electronic distribution with photons determines the optical character and spectral properties of molecules and the source of color and photochemistry. Many of the interactional forces on a large scale, including dipole and quadruple interactions, dispersion forces, and hydrogen bonding, all depend on the electronic structure of the molecules. Our appreciation of the chemistry and chemical biology of living organisms is built on an understanding of the electronic structure.