The phenomenology and theory of spin gap effects in high temperature s
uperconductors is summarized. It is argued that the spin gap behavior
can only be explained by a model of charge 0 spin 1/2 fermions which b
ecome paired into singlets and that there are both theoretical and exp
erimental reasons for believing that the pairing is greatly enhanced i
n the bilayer structure of the Y1Ba2Cu3O6+x system.