This chapter reviews some of the key experimental features of superconductivity, and outlines the Bardeen, Cooper and Schrieffer theory of conventional superconductivity. The response of a superconductor to an increasing external field can be divided into two broad categories, referred to as Type I and Type II superconductors. Nuclear magnetic resonance Knight shift measurements provided evidence for anti-parallel alignment of the hole spins, as in conventional superconductors. The results of the thermodynamic analysis place severe constraints on the possible models of superconductivity. The London equation predicts the exponential decay of a magnetic field away from the surface of a superconductor. The chapter shows how the London equation follows from the assumption of a rigid wavefunction, and describes how it leads to the Meissner effect. To explain the Meissner effect, it introduces the concept of electromagnetic momentum for a classical charged particle.