  We describe the Paired Electron Crystal (PEC) which occurs in the interacting
frustrated two-dimensional 1/4-filled band. The PEC is a charge-ordered state
with nearest-neighbor spin singlets separated by pairs of vacant sites, and can
be thought of as a bipolaron density wave. The PEC has been experimentally
observed in the insulating state proximate to superconductivity in the organic
charge-transfer solids. Increased frustration drives a PEC-to-superconductor
transition in these systems.
