THIS chapter deals with: (1) mechanical processes involving viscosity, particularly (a) impregnation, (b) breathing, (c) oil migration with respect to high voltage cables; (2) chemical processes involving viscosity, such as (a) Voltol process, (b) wax formation in cables, (c) polymerization of styrene; (3) electrical conductivity. With regard to the latter, the fundamental quantities, besides viscosity, that determine the conductivity are discussed, together with the various experimental methods whose interpretation allows the quantities in question to be computed.