An accurate and computationally tractable theoretical procedure for the calculation of the nonresonant, electronic components of the third-order molecular polarizabilities, γ(0;0,0,0), γ(−3ω;ω,ω,ω), and γ(−2ω;ω,ω,0), can be constructed. This procedure partitions γ into a σ-electron component (γσ) and a π-electron component (γπ). The γσ term is evaluated using the bond-additivity approximation; the γπ term is calculated using the semiempirical INDO all-valence-electron molecular orbital method combined with full single- and double-excitation configuration interaction (SDCI) of singlet π-electron configurations, and Orr and Ward’s sum-over-states expression for γ. The INDO-SDCI method is also used to calculate one- and two-photon spectroscopic properties of the 1ππ* states salient to γπ for the molecules of interest. It is shown that single-excitation CI alone is not sufficient for the calculation of γπ for linear polyenes and benzene. Calculations of the effect of chain length and conformation on the values of γ for ethylene, cis and trans linear polyenes, and benzene indicate that γ is strongly influenced by conjugation chain length. A simple relationship can be established between the calculated value of γπ(0;0,0,0) for the trans linear polyenes investigated and that for ethylene, the molecule with the solitary π-electron C–C bond: γπ(0;0,0,0)≂γπ(0;0,0,0)ethylene NC–C3, where NC–C=1,3,5,7, and NC–C is the total number of C=C and C–C bonds in the given polyene, i.e., the length of the π-bonding network. As γ increases with chain length, so does the ratio γπ/γ. Virtual electronic transitions involving excited π-electron states with extensive charge separation and double excited configurational character are important contributors to γπ for the linear polyenes and benzene. An approximation of γπ(0;0,0,0) for the linear polyenes can be written in terms of the linear π-electron polarizabilities for the ground state and 1 1Bu π-electron excited state. Although this approximation is strictly applicable to the centrosymmetric linear polyenes, it does suggest a very interesting criterion for the selection of organic molecules with large third-order polarizabilities. Namely, the change in polarizability between the ground state and a strongly one-photon absorbing excited state is an important factor to consider when selecting candidate molecules.