MSH 15-56 IS THE PROTOTYPICAL EXAMPLE OF AN EVOLVED COMPOSITE SUPERNOVA REMNANT (SNR) CONSISTING OF A SWEPT-UP SHELL AND A PULSAR WIND NEBULA (PWN) THAT APPEARS TO BE DISRUPTED BY THE SUPERNOVA (SN) REVERSE SHOCK (RS). GAMMA-RAY EMISSION IS DETECTED AT THE POSITION OF THE SNR THAT MAY BE PRODUCED IN THE PWN/RS INTERACTION. WE PROPOSE A 336 KS XMM OBSERVATION OF THE ENTIRE SNR CRUCIAL FOR CHARACTERIZING THE PROPERTIES OF THE AMBIENT MEDIUM THE SN EJECTA AND THE PWN AND FOR UNCOVERING THE ORIGIN OF THE HIGH-ENERGY EMISSION. THE ANALYSIS WILL ALLOW US TO TEST OUR HYDRODYNAMICAL MODELS FOR THE EVOLUTION OF COMPOSITE SNRS AND SIGNIFICANTLY ADVANCE OUR GENERAL UNDERSTANDING OF THE LATE-PHASE STRUCTURE AND EVOLUTION OF THESE SYSTEMS.