though new energy storage devices such as lithium-sulfur batteries [2] and lithium-air batteries [3] have shown great promise due to large theoretical capacity, lithium ion batteries (LIBs) are still dominating in portable electronic devices, prevailing in electric vehicles, and gradually entering grid-energy storage markets. [4] The unsatisfactory energy density of cathodes is widely recognized as the critical bottleneck for higher-performance LIBs. [5] Among various cathodes, Ni-rich layered lithium transition-metal oxides possessing a larger reversible capacity (>180 mAh g −1) than LiCoO 2 (140 mAh g −1), LiNi 1/3 Co 1/3 Mn 1/3 O 2 (160 mAh g −1), LiMn 2 O 4 (120 mAh g −1), and LiFePO 4 (160 mAh g −1), are regarded as one of the most promising cathodes for the next-generation LIBs. [6] In 2016, American electric vehicle company Tesla launched Model 3 with LiNi 0.85 Co 0.10 Al 0.05 O 2 as the cathode for its electric vehicle battery, a testament to the huge potential of Ni-rich layered oxides (NRLOs) (Scheme 1). NRLO cathodes generally consist of two main categories-LiNi 1−x−y Co x Mn y O 2 (NMC) and LiNi 1−x−y Co x Al y O 2 (NCA). The evolution from LiCoO 2 /LiNiO 2 / LiMnO 2 to LiNi 1−x−y Co x Mn y O 2 has been introduced in previous reviews. [7] Briefly, synthesizing LiCoO 2 with the low-defect density was relatively accessible due to the large difference in ionic radius between Li + and Co 3+ , but the irreversible structural change takes place when more than half a fraction of Li + is extracted from its lattice, restricting the capacity. Stoichiometric LiNiO 2 is difficult to prepare due to the instability of trivalent Ni at high temperatures, and the cation mixing between Li and Ni weakens the cycling stability of LiNiO 2. [8] Synthesis of the layered LiMnO 2 phase is not straightforward either, and the capacity fades rapidly because the structural transformation from layered to spinel phase is inevitable upon cycling. [9] LiNi 1−x−y Co x Mn y O 2 combines the strengths of nickel (high capacity), cobalt (good rate capability), and manganese (benign stability). [7] The redox couples of Ni 2+ /Ni 3+ /Ni 4+ and Co 3+ /Co 4+ contribute to the majority of the capacity. The existence of cobalt suppresses the cation mixing during the synthesis of stoichiometric compounds, while manganese helps stabilize the Ni-rich layered oxides (NRLO) are widely considered among the most promising cathode materials for high energy-density lithium ion batteries. However, the high proportion of Ni content accelerates the cycling degradation that restricts their large-scale applications. The origins of degradation are indeed heterogeneous and thus there are tremendous efforts devoted to understanding the underlying mechanisms at multi-length scales spanning atom/lattice, particle, porous electrode, solid-electrolyte interface, and cell levels and mitigating the degradation of the NRLO. This review combines various advanced in situ/ex situ analysis techniques developed for resolving NRLO degradation at multi-length scales and aims...