The aging mechanisms of Li-ion batteries are introduced in this chapter, and are experimentally investigated and modeled. From SEM it is found that the thickness of the solid electrolyte interface layers at the graphite electrode surface increase upon aging. Deformation of the graphite structure is confirmed by Raman spectroscopy. XPS analyses show that transition metals dissolved from cathode are deposited onto the graphite electrode. Cathode dissolution at elevated temperatures is further confirmed by ICP measurements. Apart from postmortem analyses, a novel non-destructive approach is proposed to quantify the graphite electrode decay. A comprehensive electrochemistry model is proposed to simulate the irreversible capacity loss under various aging conditions. The dependence of the capacity loss on aging conditions, such as storage state of charge, cycling current, temperature, etc. is simulated and the simulations are in good agreement with the experiments. The degradation model allows researchers to have an in-depth understanding of aging mechanisms and therefore helps manufacturers to improve battery performance by optimizing manufacturing procedures. Moreover, the model can be further used to predict the battery cycle life, which can be used to develop more accurate battery management systems to increase battery efficiency and safety.