Research on rechargeable lithium ion batteries has its 40th anniversary this year. The first batteries used LiAl anodes and TiS2 cathodes, and the first highly commercially successful cells used LiC6 and LiCoO2 electrodes. These latter cells are now 20 years old, and their energy storage capability has significantly improved over the years. However, they still attain only around 20% of the theoretical energy density, either on a weight or volume basis [IEEE Proceedings, 100, 1518, 2012]. This is in part a result of the very low volumetric capacity of the carbon anode, but also the inability to use much more than 60% of the lithium capacity in the layered oxides at practical rates. A fundamental understanding of the electrode reactions is essential, and will lead to higher rates and capacities; higher rates will allow the use of thicker electrodes and therefore less current collectors and separators. The olivine materials and layered oxides will be used as examples where our understanding is improving. The former represent a class of material, which despite being electronic insulators, can sustain high rates and high capacity retention. This can be related to a pseudo-single phase reaction mechanism, which is enhanced by substitution on the iron site. The latter are challenged by the high mobility of the transition metal ions at room temperature, which allows the metal distribution to change on cycling often in an irreversible manner. This work is supported by the US Department of Energy through the NECCES-EFRC program. Visit Dr. M. Stanley Whittingham's website to learn more.