The main properties of conventional, metallic superconductors are presented. The electrodynamic behavior of type I superconductors is well described by the London theory. The thermodynamic properties in the normal and the superconducting state are compared. Some of the most important ideas and results of the BCS theoryBCS theory are briefly presented. Flux quantization, tunneling experiments and macroscopic quantum interferenceQuantum interference effects reflect that superconductivity is a macroscopic quantum phenomenon. The differences of the magnetic behavior of type I and type II superconductors are described. Finally, the anisotropy of the properties of superconductors with a layered crystal structure is considered.