Currently, the most energy-dense lithium-ion batteries are based on rock-salt-type cathodes, and improving on the prior art without sacrificing lifetime has been a formidable challenge. Here I will review our current understanding of structure-composition-stability/capacity relationships in rock-salt-type electrodes, obtained from first-principles calculations, percolation theory, model Hamiltonian analysis, computational phase diagrams, and machine learning. In layered oxides with the general composition LiMO<sub>2</sub> (M = one or more metal species) in which lithium ions are segregated into separate layers, Li/M disorder leads to the loss of reversible capacity. In contrast, in Li-rich compositions Li<sub>1+x</sub>M<sub>1-x</sub>O<sub>2</sub>, Li/M disorder can be advantageous. A common mode of degradation in both layered and disordered oxides is the release of oxygen from the surface of the electrode at high voltages or high temperatures, which coincides with the formation of surface reconstructions. I will discuss similarities/differences of ordered and disordered cathodes and point out open questions.