Cadmium (Cd) contamination poses a significant threat to ecosystems and human health. Cd stable isotope analysis is increasingly used to identify contaminant sources and track Cd redistribution, but its environmental interpretation remains challenging because source signatures can be modified by isotope fractionation during transport. This review synthesizes recent progress in Cd isotope methods, fractionation mechanisms, environmental reservoirs, source tracing, and cross-media transfer. We organize published evidence around a mechanistic pathway from analytical expression and mass-bias correction to isotope fractionation during evaporation-condensation, biological uptake, adsorption, precipitation, leaching, and soil-plant transport. We further compare isotope behavior in ores, soils, sediments, waters, and biota, and evaluate how isotope signals are preserved or overprinted across rock-soil, water-soil, and soil-plant interfaces. The synthesis shows that Cd isotopes are most reliable for source apportionment when endmembers are isotopically distinct, concentration-isotope mass balances are constrained, and post-source fractionation is explicitly considered. Conversely, overlapping source signatures, limited endmember databases, and incomplete process-specific isotope fractionation offsets (Δ^114/110Cd) remain major obstacles.