Lithium-ion battery degradation limits the performance, safety, cost, and service life of consumer electronics, electric vehicles, industrial systems, and grid-energy storage. This review synthesizes the electrochemical, mechanical, thermal, and transport processes that govern battery ageing and links them to health assessment, lifetime prediction, and battery-management decisions. A mechanism-to-decision framework is used to distinguish degradation mechanisms, degradation modes, and cell-level outcomes. The review examines solid-electrolyte-interphase growth, lithium plating, particle fracture, cathode reconstruction, transition-metal dissolution, electrolyte decomposition, separator damage, and electrode cross-talk. These processes contribute to lithium loss, active-material loss, electrolyte depletion, stoichiometric imbalance, impedance growth, capacity fade, and power fade. The effects of temperature, state of charge, depth of discharge, current rate, ageing history, chemistry, and cell design are assessed, with emphasis on coupled mechanisms, spatial non-uniformity, path dependence, and knee-point ageing. The review also discusses state-of-health estimation, remaining-useful-life prediction, mitigation strategies, and digital-twin approaches. Capacity alone is insufficient because similarly aged cells may differ in resistance, safety margin, and future lifetime. Future research should prioritize realistic pack-level datasets, uncertainty-aware prediction, standardized testing, operando diagnostics, transferable models, and decision-oriented battery management.