The mechanism responsible for superconductivity in the copper oxides is still a matter of serious debate. This section provides a qualitative comparison between the s-wave pairing state associated with conventional metallic superconductors and a proposed d-wave pairing mechanism for the high-temperature superconductors. The latter has attracted considerable attention in the past few years because it is experimentally verifiable. However, conflicting experimental results have left the nature of the pairing mechanism still very much a mystery. This chapter will avoid the rigorous mathematical details associated with each mechanism; the interested reader is referred to references within.