GALAXY CLUSTERS ARE THE LARGEST VIRIALIZED STRUCTURES IN THE UNIVERSE. THEIR SPATIAL DISTRIBUTION AND NUMBER DENSITY CARRIES THE IMPRINT OF THE PROCESS OF STRUCTURE FORMATION AND AS A CONSEQUENCE THESE PROPERTIES ARE SENSITIVE TO THE UNDERLYING COSMOLOGICAL PARAMETERS. IN ORDER TO USE CLUSTER NUMBER COUNTING TO CONSTRAIN THE COSMOLOGICAL PARAMETERS THE TOTAL CLUSTER MASS NEEDS TO BE DETERMINED WITH HIGH PRECISION. MULTI-WAVELENGTH DATA FOR UNBIASED SAMPLES ALLOW US TO SIGNIFICANTLY IMPROVE MASS MEASUREMENTS AND THUS ACCURATELY DETERMINE THE CLUSTER MASS FUNCTION. OUR XMM-HERITAGE PROGRAM WAS AWARDED 3 MS TO STUDY A RIGOROUSLY DEFINED SAMPLE OF CLUSTERS OF GALAXIES DETECTED BY PLANCK THROUGH THEIR SUNYAEV-ZEL'DOVICH EFFECT TO (I) OBTAIN AN ACCURATE VISION OF THE STATISTICAL PROPERTIES OF THE LOCAL CLUSTER POPULATION AND IN THE HIGHEST MASS REGIME; (II) UNCOVER THE PROVENANCE OF NON-GRAVITATIONAL HEATING; (III) MEASURE HOW THEIR GAS IS SHAPED BY THE COLLAPSE INTO DARK MATTER HALOES; (IV) RESOLVE THE MAJOR UNCERTAINTIES IN MASS DETERMINATIONS THAT CURRENTLY LIMIT COSMOLOGICAL INFERENCES; (V) BUILD THE FOUNDATION FOR CLUSTER SCIENCE WITH NEXT-GENERATION SURVEYS. WE HAVE A UNIQUE AND TIMELY OPPORTUNITY TO DEVELOP ALL THE NECESSARY TOOLS TO RESOLVE THE MAJOR UNCERTAINTIES IN THE CLUSTER MASS DETERMINATION THAT ARE CURRENTLY LIMITING COSMOLOGICAL INFERENCES. OUR TEAM BRINGS TOGETHER THE EXPERIENCE AND EXPERTISE OF TOP LEVEL RESEARCHERS FOR THE ANALYSIS OF ARCHIVAL DATA AND OF THE NEWLY AWARDED XMM-NEWTON OBSERVATIONS. HERE WE PROPOSE CLEAR AND DETAILED PROJECTS TO INVESTIGATE NEW METHODS TO ADDRESS SPECIFIC PROBLEMS AND TO DETERMINE THE BEST POSSIBLE ANALYSIS STRATEGY TO APPLY TO THE FINAL CLUSTER SAMPLE THAT WILL PROVIDE A SOUND BASIS TO MEASURE THE ULTIMATE EVOLUTION OF GALAXY CLUSTERS. THIS PROPOSAL WILL SUPPORT THE U.S. PARTICIPATION IN THIS LARGE INTERNATIONAL COLLABORATION. OUR ROLE IN THIS PROJECT WILL BE TO DETERMINE THE CURRENT SYSTEMATIC UNCERTAINTIES IN THE ESTIMATES OF X-RAY MASS AND OTHER GLOBAL PROPERTIES BY COMPARING THE RESULTS FROM DIFFERENT XMM-NEWTON ANALYSIS PIPELINES FOR A SAMPLE OF GALAXY CLUSTERS. THIS ANALYSIS WILL BE CONDUCTED SYSTEMATICALLY AND SELF-CONSISTENTLY ON A WELL DEFINED AND UNBIASED CLUSTER SAMPLE. WE WILL NOT ONLY PROVIDE THE COMMUNITY WITH THE CURRENT LEVEL OF UNCERTAINTIES IN THE X-RAY ANALYSIS BUT ALSO DEVELOP THE BEST AND MOST UP-TO-DATE ANALYSIS METHODS WHICH WILL COMBINE THE MOST ADVANCED APPROACH OF EACH XMM-NEWTON ANALYSIS PIPELINE (E.G. PSF CORRECTION BACKGROUND MODELING DEPROJECTION METHOD). PARTICULAR FOCUS WILL BE DEDICATED TO DEVELOP THE STRATEGY THAT WILL BE APPLIED TO THE NEARBY SYSTEMS THAT REQUIRES THE PARALLEL ANALYSIS OF MULTIPLE OBSERVATIONS. WHILE A CONSISTENT PICTURE OF CLUSTER MASS ESTIMATES WILL BE ACHIEVED FROM THE FINAL SAMPLE BY COMBINING ALL THE MULTI-WAVELENGTH INFORMATION THERE ARE MANY STEPS THAT MUST BE DONE NOW. STARTING FROM WELL-DEFINED HERITAGE CLUSTER SUB-SAMPLES WE WILL EVALUATE THE DIFFERENT BIASES AFFECTING THE SCALING RELATIONS INCLUDING SELECTION EFFECTS THE ROLE OF DISTURBED SYSTEMS AND THE REPRESENTATIVENESS OF DIFFERENT CLUSTER SAMPLES (E.G. SZ AND X-RAY SELECTION). FOR THE FIRST TIME WE PLAN TO INCLUDE IN THIS ANALYSIS ALL THE INFORMATION (E.G. FLUCTUATION AND SCATTER LEVELS) ABOUT THE CLUSTER DYNAMICAL STATES THAT IS EMBODIED IN THE THERMODYNAMICAL 2D MAPS. WE WILL PERFORM A SIMULTANEOUS ANALYSIS OF XMM-NEWTON AND PLANCK DATA BY ADAPTING JACO A CODE EMPLOYING PARAMETRIC MODELS FOR BOTH THE DARK-MATTER DENSITY AND GAS DENSITY INTO THE FITTING PROCEDURE TO THE XMM-NEWTON DATA AND TAKE INTO ACCOUNT UPDATED BACKGROUND AND PSF TREATMENTS AND CALIBRATIONS.