This chapter shows how the theories of molecular structure lead to an understanding of the strengths of bonds and other physical properties of organic compounds. The two most commonly employed methods for describing the structures of organic compounds, each of which is related to the theories of wave mechanics, are the molecular-orbital (MO) and valence-bond (VB) methods. In MO treatment, bonding is described as arising from the overlap of atomic orbitals (AOs) of the atoms involved, leading to the formation of molecular orbitals (MOs). The interaction of one AO on one atom with one on another gives two MOs, of which one is a bonding orbital and the other is an antibonding orbital. The extra stability of conjugated compounds compared with non-conjugated compounds, as measured, for example, by heats of reduction, is not entirely attributable to delocalization. The concept of bond moments can be usefully extended to establish a scale of inductive effects.