A very attractive way to motivate R-parity is with a B − L continuous gauge symmetry which is spontaneously broken at a scale inaccessible to current experiments. A continuous U ( 1 ) B − − L forbids renormalizable terms which violate B and L. If U ( 1 ) B − − L is only broken by scalar vacuum expectation values (or other order parameters) that carry even integer values of 3(B − L), then there exist an exactly conserved discrete remnant subgroup which has the desired properties. The crucial issue is to determine whether the sneutrino (the supersymmetric partner of neutrino), which is odd under R-parity, develops a vacuum expectation value. It can be shown, on phenomenological grounds, that this cannot happen in any theory where U ( 1 ) B − − L is broken at a scale much above the electroweak one. This is true in any theory based on a large-scale seesaw mechanism. As a consequence, in such theories R-parity remains exact at all energies.