It is demonstrated that the transition temperature (TC) ofhigh-TC superconductors is determined by their layered crystal structure, bondlengths, valency properties of the ions, and Coulomb coupling betweenelectronic bands in adjacent, spatially separated layers. Analysis of 31 high-TC materials (cuprates, ruthenates, ruthenocuprates, iron pnictides, organics) yields theuniversal relationship for optimal compounds, kBTC0 = β/ℓζ, where ℓ is related to the mean spacing between interacting charges in the layers,ζ is the distance between interacting electronic layers,β is a universalconstant and TC0 is the optimal transition temperature (determined to within an uncertainty of ± 1.4 K by this relationship). Non-optimum compounds, in which sample degradation is evident, e.g. bybroadened superconducting transitions and diminished Meissner fractions, typically exhibit reducedTC < TC0. It is shownthat TC0 may be obtained from an average of the Coulomb interaction forces between the two layers.