BCS Theory, formulated by Bardeen, Cooper, and Schrieffer in 1957, is the first microscopic theory explaining superconductivity, describing how electron pairs, known as Cooper pairs, form and move without resistance through a crystal lattice. This theory revolutionized the understanding of low-temperature physics by introducing the concept of an energy gap in the electronic density of states, accounting for the phenomenon of zero electrical resistance and the expulsion of magnetic fields in superconductors.