THE PURPOSE OF THIS PROPOSAL IS TO IDENTIFY A POPULATION OF COMPACT SUPERMASSIVE BLACK HOLE BINARIES (SMBHBS) IN GALACTIC NUCLEI. SMBHBS ARE BELIEVED TO FORM FREQUENTLY IN GALAXY MERGERS AND ARE A FUNDAMENTAL INGREDIENT IN THE HIERARCHICAL ASSEMBLY OF GALAXIES. THEY ARE ALSO PRIME TARGETS FOR ON-GOING AND PLANNED GRAVITATIONAL WAVE EXPERIMENTS DESIGNED TO TEST OUR FUNDAMENTAL UNDERSTANDING OF GRAVITY. WE WILL ANALYZE ARCHIVAL UV X-RAY AND IR OBSERVATIONS TO TEST THE BINARY HYPOTHESIS FOR A SAMPLE OF APPROXIMATELY 150 QUASARS NEWLY IDENTIFIED AS BINARY CANDIDATES BASED ON PERIODICITIES IN THEIR OPTICAL LIGHT-CURVES. WE WILL TEST THE RELATIONS PREDICTED BY BINARY MODELS BETWEEN THE OPTICAL UV AND X-RAY PERIODICITES (ALL DIRECTLY PRODUCED IN THE ACCRETION DISKS NEAR THE BHS) AND THEIR IR LIGHT ECHOES (PRODUCED FARTHER OUT IN A DUSTY CIRCUMBINARY TORUS). WE WILL CAREFULLY COMPARE OUR FINDINGS WITH A SUITABLY DESIGNED CONTROL SAMPLE OF NON-PERIODIC QUASARS WITH SIMILAR PROPERTIES. WE WILL INTEPRET THE INFERRED ABUNDANCE OF BINARIES BY COMPARING THEM TO THEORETICAL MODELS FOR THE UNDERLYING SMBHB POPULATION. FINALLY WE WILL PERFORM FOLLOW-UP OPTICAL OBSERVATIONS OF THE MOST PROMISING CANDIDATES. THIS PROPOSAL WILL SUBSTANTIALLY LEVERAGE RECENT THEORETICAL WORK INCLUDING PREDICTIONS BY OUR OWN GROUP FOR PERIODIC VISIBLE UV AND X-RAY SIGNALS FROM BINARIES THROUGH HYDRODYNAMICAL ACCRETION RATE FLUCTUATIONS AND THROUGH RELATIVISTIC DOPPLER BOOSTING. THE RESULTS WILL BE APPLIED TO SMBHB CANDIDATES IDENTIFIED OVER THE PAST YEAR. OUR PROPOSAL WILL DIRECTLY SUPPORT NASA'S STRATEGIC PLAN SUB- GOAL 3D: "DISCOVER THE ORIGIN STRUCTURE EVOLUTION AND DESTINY OF THE UNIVERSE AND SEARCH FOR EARTH-LIKE PLANETS".