Abstract: Nitrogen oxide (NO_x) emissions are a critical contributor to mobile‑source pollution, which is particularly severe during the cold‑start phase of diesel vehicles, posing substantial threats to the environment and human health. Passive NO_x adsorber (PNA) technology represents an effective strategy to achieve low‑NO_x emissions from diesel vehicles. This review systematically summarizes the latest advances in the synthesis design and mechanistic investigation of metal‑based PNA materials. Particular emphasis is placed on the structure‑activity relationships and modification strategies of cerium‑based metal oxides, while other non‑noble metal systems including cobalt‑based, manganese‑based, and composite metal oxides are also covered. Two core mechanisms are elaborated in detail: the adsorption–oxidation–desorption mechanism wherein NO_x is stored and decomposed in the forms of nitrites/nitrates, and the adsorption–complexation–desorption mechanism involving NOx storage and release via metal‑ion complex formation. Furthermore, the effects of other components in exhaust (H₂O, CO, and hydrocarbons), hydrothermal aging, and chemical poisoning induced by sulfur and phosphorus on the NOx adsorption–desorption behaviors of PNA materials are discussed. Finally, the major challenges and future research directions for metal‑based PNA materials are proposed.