  We revisit the interlayer tunneling theory of high temperature
superconductors and formulate it as a mechanism by which the striking
systematics of the transition temperature within a given homologous series can
be understood. We pay attention not only to the enhancement of pairing, as was
originally suggested, but also to the role of competing order parameters that
tend to suppress superconductivity, and to the charge imbalance between
inequivalent outer and inner CuO2 planes in a unit cell. Calculations based on
a generalized Ginzburg-Landau theory yield results that bear robust and
remarkable resemblance to experimental observations.
