Stable water isotopes (δ¹⁸O and δD) preserved in ice cores record past climate change. Reconstructing historical temperatures from ice cores relies fundamentally on the empirical linear relationship between stable water isotopes and local temperature. However, post-depositional processes at the glacier surface continuously alter initial isotopic signatures across diverse climatic regimes, yet their differential contributions across regions remain poorly understood. In this review, we synthesise findings from current field observations, laboratory experiments, and numerical modelling to elucidate the physical mechanisms governing isotopic evolution at the glacier surface. We systematically evaluate key post-depositional drivers, including sublimation–deposition, turbulent mixing, solid- and gas-phase diffusion, dry- and wet-snow metamorphism, wind-driven redistribution and pumping, and meltwater infiltration–refreezing. Previous observations from East Antarctica indicate post-depositional enrichments in near-surface δ¹⁸O of 3.0–3.3 ‰ and depletions in <em>d</em>-excess of 3.4–3.5 ‰, while Greenland surface snow exhibits diurnal fluctuations of ~2.5 ‰ in δ¹⁸O and ~13 ‰ in δD. These modifications reduce seasonal signals and weaken the local isotope-temperature relationship. We emphasise that future high-resolution field observations of isotopic compositions within snowpacks and atmospheric moisture must be prioritised. Such observational efforts are essential to elucidate key physical mechanisms, optimise the isotope-enabled atmosphere–snowpack models, and enhance our capacity to simulate interfacial isotopic exchange. Finally, we identify key research priorities to bridge existing knowledge gaps and propose an integrated observation-experiment-modelling framework to improve the calibration of water isotope proxies and enhance the precision of ice-core-based paleoclimatic reconstructions.