The shielding factor of a cable buried in the ground has been investigated, and it is shown that it may be considered in terms of three components. The first of these depends on the sheath construction only, and is the factor normally considered for high-frequency shielding. The second component is dependent on the sheath and its associated earth-return circuit, for very long lengths of cable. The third component, which it is believed has not been generally considered previously, depends on the length and terminating impedances of the sheath-to-earth circuit. A theoretical treatment is given for these three components. Formulae are quoted for the first two, and a formula is derived for the third. Experimental investigation of the first component has been made using short telephone cables with non-magnetic homogeneous sheaths, and theoretical correction has been made for their use in earth of medium resistivity.