The Bardeen-Cooper-Schrieffer Theory, often abbreviated as BCS Theory, is a fundamental explanation of superconductivity, a phenomenon where certain materials can conduct electricity without resistance at low temperatures. Developed in 1957 by John Bardeen, Leon Cooper, and Robert Schrieffer, the theory describes how electrons form pairs, known as Cooper pairs, which move through a lattice structure without scattering. These Cooper pairs arise due to attractive interactions between electrons mediated by lattice vibrations, or phonons. The BCS Theory successfully explains various properties of superconductors, including the energy gap and critical temperature, and has significantly advanced our understanding of quantum mechanics and condensed matter physics.