The technical literature, including datasheets from battery manufacturers, is reviewed to identify the major characteristics of capacity fade: impedance growth and capacity loss. Impedance growth is significant at both the positive and negative electrodes, though capacity fade loss due to SEI growth is usually attributed to the negative. Techniques for accelerated life testing are summarized. Simple models are used to describe mechanisms for capacity loss at the negative. Finally, numerical simulations are used to explore the effect of porous electrodes on fade behavior.