  Seven distinct families of superconductors with critical temperatures at
ambient pressure that equal or surpass the historic 23 K limit for Nb3Ge have
been discovered in the last 25 years. Each family is reviewed briefly and their
common chemical features are discussed. High temperature superconductors are
distinguished by having a high (\geq 50%) content of nonmetallic elements and
fall into two broad classes. 'Metal-nonmetal' superconductors require a
specific combination of elements such as Cu-O and Fe-As which give rise to the
highest known Tc's, probably through a magnetic pairing mechanism.
'Nonmetal-bonded' materials contain covalently-bonded nonmetal anion networks
and are BCS-like superconductors. Fitting an extreme value function to the
distribution of Tc values for the known high-Tc families suggests that the
probability of a newly discovered superconductor family having maximum Tc > 100
K is ~0.1-1%, decreasing to ~0.02-0.2% for room temperature superconductivity.
