BCS theory successfully explains low temperature superconductivity, the ability of certain materials at low temperatures to conduct electricity without resistance. BCS theory views superconductivity as a macroscopic quantum mechanical effect. It proposes that electrons with opposite spin can become paired, forming Cooper pairs. An electron moving through a conductor will cause a slight increase in concentration of positive charges in the lattice around it; this increase in turn can attract another electron. In effect, the two electrons are then held together with a certain binding energy. If this binding energy is higher than the energy provided by kicks from oscillating atoms in the conductor (which is true at low temperatures), then the electron pair will stick together and resist all kicks, thus not experiencing resistance.
BCS theory predicts that above a temperature of 30-40 K, Cooper pair superconductivity cannot exist. In 1986, this high temperature superconductivity was however discovered, implying that at these temperatures other effects are at play; these effects are not yet fully understood.