Lithium-ion cells containing lithium and manganese rich layered-oxides (LMR-NMC) have gained significant attention in recent years because of their ability to deliver high energy densities. In this article we report on a comprehensive performance and degradation study of cells, containing Li1.2Ni1.5Mn0.55Co0.1O2 based positive electrodes and graphite based negative electrodes, on extended cycling. In addition to electrochemical measurements on full cells, characterization data on harvested electrodes by techniques that include scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), Raman spectroscopy, and secondary ion mass spectrometry (SIMS) are discussed. Our data show that cell capacity fade mainly results from lithium trapping in the solid electrolyte interphase (SEI) of the negative electrode. In addition, cell impedance rise and voltage fade mainly arise at the positive electrode and result from degradation processes in its oxide and carbon constituents. Processes that include the accumulation of transition metal elements at the negative electrode, and increasing misalignment of electrode capacity windows on extended cycling, also have a deleterious effect on cell performance. Identifying sources of performance degradation has enabled strategies to extend cell life, which include improved cell fabrication protocols, positive electrode coatings, and bifunctional electrolyte additives. (C) 2013 The Electrochemical Society.