The substituent shell concept is that, for a series of compounds I G-Y with a common class of group C, the effects of changing X on various properties of Y should be linearly related. The concept is examined theoretically and empirically. The results of Huckel, and of ab intio SCF, MO calculation of electron populations are analysed In terms of a recently developed perturbational molecular orbital model, called Polarisation and Charge-Transfer (PCT) model, for the transmission of substituent effects. It is seen that the individual substituent effects of E on the electronic structure of G, can be blended by transmission through C, to produce a single-valued composite substituent effect on the electronic structure of Y, in accord with the substituent shell concept. A number of empirical data sets such as Qp, or _p, d +m etc., are tested and shown to be in accord with the concept, at least for limited sets of E.The way in which substituent effects on properties of Y vary with the nature of the solvent is also studied. Empirical data sets andsolvent scales such as q, . Taft's P ,7r , B, and DN, are examined by the statistical technique of Factor Analysis, and some physically significant contributions to the total solvent effect are identified.