WE ARE PROPOSING TO STUDY A PROBE-SCALE MISSION TO EXTRACT THE WEALTH OF PHYSICAL COSMOLOGICAL AND ASTROPHYSICAL INFORMATION CONTAINED IN THE SPECTRUM AND POLARIZATION OF THE COSMIC MICROWAVE BACKGROUND (CMB). THE CMB PROBE WILL SEARCH FOR THE SIGNATURE OF PRIMORDIAL GRAVITATIONAL WAVES FROM THE BIG BANG AND THUS PROBE QUANTUM GRAVITY. IT WILL CONSTRAIN THE EFFECTIVE NUMBER OF LIGHT PARTICLE SPECIES WITH PRECISION ONLY AVAILABLE TO CMB MEASUREMENTS. WITH ITS FULL SKY COVERAGE IT WILL MEASURE THE SUM OF THE NEUTRINO MASSES DOUBLING THE SIGNIFICANCE OF A 2 SIGMA DETECTION REACHABLE BY EXPERIMENTS THAT MEASURE ONLY SMALLER PORTIONS OF THE SKY. IT WILL PROBE THE NATURE OF DARK MATTER AND THE EXISTENCE OF NEW FORMS OF MATTER IN THE EARLY UNIVERSE. IT WILL GIVE NEW INSIGHTS ON THE STAR-FORMATION HISTORY ACROSS COSMIC TIMES. AND IT WILL PROVIDE INFORMATION ABOUT THE PROCESSES THAT CONTROL STRUCTURE FORMATION ON ALL SCALES FROM CLUSTERS OF GALAXIES TO THE COLLAPSE OF A PROTOSTELLAR CORE. WITH HIGH SENSITIVITY ACCESS TO THE ENTIRE SKY BROAD FREQUENCY COVERAGE AND EXQUISITE CONTROL OF SYSTEMATIC EFFECTS THE PROBE IS BEST POISED TO REALIZE THE FIDELITY OF MEASUREMENTS NECESSARY TO EXTRACT THESE SCIENCE GOALS. THE LAST US CMB COMMUNITY'S CONSENSUS ASSESSMENT OF THE CASE FOR AND DESIGN OF A SPACE MISSION TOOK PLACE 8 YEARS AGO. SINCE THEN THEORETICAL CONSIDERATIONS AVAILABLE DATA FROM PLANCK AND SUB-ORBITAL MEASUREMENTS TECHNOLOGY ADVANCES AND PLANS FOR NEW SUB-ORBITAL EXPERIMENTS HAVE CHANGED THE LANDSCAPE CONSIDERABLY. WE PROPOSE TO PROVIDE THE 2020 DECADAL PANEL WITH A FRESH EXPERT ASSESSMENT. THE SCOPE OF SCIENCE WE ENVISION FOR THE PROBE IS ACHIEVABLE WITHIN THE APPROXIMATE TECHNICAL ENVELOPE OF OUR 2010 BASELINE MISSION WHICH WAS NEAR $900M. THIS SCOPE OF SCIENCE IS ALSO TARGETED BY A RECENTLY SUBMITTED PROPOSAL FOR A EUROPEAN-BASED MISSION THAT HAS SIMILAR COST. BOTH OF THESE MISSIONS HAVE BROADER SCIENCE REACH THAN A MORE FOCUSED JAPANESE-LED MISSION WHICH IS NEAR THE $400M LIMIT. WE THUS ASSESS THAT THE CMB MISSION IS IN THE PROBE COST WINDOW. THE MISSION STUDY IS LED BY STEERING AND EXECUTIVE COMMITTEES MADE UP OF SCIENTISTS WHO BUILT COBE WMAP PLANCK AND THE LEADING SUB-ORBITAL EXPERIMENTS IN THE WORLD; BY SCIENTISTS WHO PROCESSED ANALYZED AND SIMULATED DATA FROM THESE EXPERIMENTS; AND WHO INTERPRETED THE RESULTS AND PUT THEM IN A PHYSICS AND COSMOLOGY CONTEXT. IT IS OPEN TO ALL MEMBER OF THE CMB COMMUNITY AND WILL REPRESENT HUNDREDS OF PERSON YEARS OF ACCUMULATED KNOWLEDGE EXPERTISE AND EXPERIENCE.