In this chapter, we review the basic properties of superconductors, illustrating that they go beyond describing a metal without resistance. We develop London's mesoscopic theory of superconductivity as a quantum mechanical description of the charged superfluid, which accounts for the behavior of the superconductor under electromagnetic fields. Using this theory, we explain the flux quantization for supercurrents flowing in closed loops and derive the energetics of the Josephson junction from the tunneling of Cooper pairs.