QUASARS AND OTHER ACTIVE GALACTIC NUCLEI PLAY A KEY ROLE IN OUR UNDERSTANDING OF THE PHYSICAL UNIVERSE AND ITS MAJOR CONSTITUENTS.THEY ARE INTERESTING ASTROPHYSICAL PHENOMENA IN THEIR OWN RIGHT AND A PROBE OF RELATIVISTIC PHYSICS. THEY CO-EVOLVE WITH THEIR HOST GALAXIES AND TRACE THE EVOLUTION OF COSMIC STRUCTURE. OUR INSIGHTS INTO THEIR PHYSICS AND EVOLUTION KEEP IMPROVING THANKS IN LARGE PART TO SYSTEMATIC SURVEYS SPANNING A BROAD RANGE OF WAVELENGTHS BOTH FROM THE GROUND AND FROM SPACE. VARIABILITY OF QUASARS IS ONE OF THEIR KEY OBSERVABLE CHARACTERISTICS AND IT IS PRESENT AT ALL WAVELENGTHS AND AT ALL TIME SCALES PROBED SO FAR. IT REFLECTS DIRECTLY THE PHYSICS OF THEIR FUELING AND BEAMING AND CAN THUS BE USED TO PROVIDE UNIQUE OBSERVATIONAL CONSTRAINTS TO THEORETICAL MODELS. THE FIELD IS BEING TRANSFORMED BY THE AVAILABILITY OF MASSIVE DATA SETS FROM SYNOPTIC SKY SURVEYS BOTH FROM THE GROUND AND SPACE THAT COVER A LARGE PORTION OF THE SKY REPEATEDLY OVER A BROAD RANGE OF TIME BASELINES. SOME OF THE RECENT RESULTS INCLUDE:* A DISCOVERY OF A CHARACTERISTIC TIME SCALE FOR A STOCHASTIC VARIABILITY OF QUASARS THAT ANTI-CORRELATES WITH THEIR LUMINOSITY AND THUS THE PHYSICS OF THEIR ACCRETION DISKS WHICH IS STILL NOT FULLY UNDERSTOOD.* A DISCOVERY OF PERIODIC VARIABILITY IN QUASAR LIGHT CURVES (FOR APPROX. 1 IN 3 000 OBJECTS) AS A LIKELY SIGNATURE OF CLOSE BINARY SUPERMASSIVE BLACK HOLES (SMBH) EN ROUTE TO A MERGER. WITH THE ESTIMATED MILLIPARSEC SEPARATIONS THESE BINARIES ARE SPATIALLY UNRESOLVABLE BY ANY ANTICIPATED TECHNOLOGY AND VARIABILITY REPRESENTS THE ONLY WAY OF PROBING THIS POPULATION. THESE ARE THE PROGENITORS OF THE EXPECTED SOURCES OF LONG-WAVELENGTH GRAVITATIONAL WAVE BURSTS THAT SHOULD RESULT FROM SUCH SMBH MERGERS ONE OF THE KEY PREDICTIONS OF THE MODELS OF THEIR HIERARCHICAL ASSEMBLY AND EVOLUTION. THE CURRENT SAMPLE FAVORS A POPULATION OF AN UNEVEN MASS RATIO (Q=0.01) BINARIES WITH THE PRESENCE OF GAS WHICH WOULD SPEED UP THE FINAL MERGER PHASE.* DISCOVERIES OF A NUMBER OF DISTINCT SUBPOPULATION OF QUASARS SHOWING UNUSUAL OR EXTREME VARIABILITY PATENTS INCLUDING:* MAJOR LONG-DURATION FLARE EVENTS SOME BUT NOT ALL OF WHICH MAY BE RELATED TO A NEW POPULATION OF ULTRALUMINOUS SNE FROM THE ACCRETION DISKS OR SOME HERETOFORE UNKNOWN ENERGETIC PHENOMENA ASSOCIATED WITH THE ACTIVE NUCLEI.* CHANGING SPECTROSCOPIC TYPE (E.G. FROM TYPE 1 TO TYPE 2) AGN WHICH MAY RESULT FROM THE CHANGES IN OBSCURATION ALONG THE LINE OF SIGHT AND/OR THE LUMINOSITY CHANGES FROM THE CENTRAL ENGINE.VARIABILITY ALSO OFFERS A SPECTRUM-INDEPENDENT METHOD FOR QUASAR DISCOVERY THAT BYPASSES MANY SELECTION EFFECTS AND THUS A TEST OF THE COMPLETENESS OF THE EXISTING SAMPLES. COMBINING THE VARIABILITY-BASED AND SPECTRUM-BASED SELECTION CRITERIA WOULD LEAD TO MORE COMPLETE SAMPLES OF QUASARS THAT WOULD AFFECT OUR UNDERSTANDING OF THEIR PHENOMENOLOGY AND EVOLUTION.WE PROPOSE TO COMBINE MULTI-WAVELENGTH AND MULTI-EPOCH DATA SETS FROM A VARIETY OF MISSIONS FROM MID-IR TO GAMMA-RAY SUPPLEMENTED WITH MULTICOLOR OPTICAL NIR AND RADIO SURVEYS FROM THE GROUND IN ORDER TO FURTHER EXPLORE AND INTERPRET THESE FINDINGS AND USE THEM TO PROVIDE NEW OBSERVATIONAL CONSTRAINTS FOR THEORETICAL MODELS THAT MAY LEAD TO NEW INSIGHTS INTO THE PHYSICS AND EVOLUTION OF AGN.