BCS theory (J. Bardeen, L.N. Cooper, J.R. Schrieffer; 1957)
A theory put forth to describe superconductivity and superfluidity both. This recommend that in the superconducting (or superfluid) state electrons form Cooper pairs, where two electrons behave as a single unit. This takes a nonzero amount of energy to break such kind of pairs, and the imperfections in the superconducting solid (which would lead to resistance normally) are incapable of breaking the pairs, thus no dissipation takes place and there is no resistance.