The chemistry of high-valent metal species (HVMs) generated via Fenton (-like) reactions has been extensively reported in the literature. HVMs are recognised as highly reactive yet selective oxidants, in contrast to hydroxyl radicals—one of the most potent but nonselective oxidants—exhibiting distinct reactivity towards a broad range of organic compounds. Owing to their target-specific behaviour, HVM-based oxidation processes have garnered considerable attention for applications in water and wastewater treatment. This review focuses on ferryl and trivalent copper species (i.e. FeIVO2+ and Cu(III)), which are among the most environmentally relevant and widely studied HVMs. The formation mechanisms of these species via Fenton (-like) reactions are examined, followed by a detailed discussion of their chemical properties, including structural features, redox potentials, and hydrolytic behaviours. The unique reactivity of HVMs towards diverse organic molecules is then discussed in relation to substrate structures and corresponding oxidation mechanisms. Analytical methods for detecting FeIVO2+ and Cu(III) are also reviewed, including chemical probe techniques, scavenger-based inhibition assays, electron paramagnetic resonance, and other complementary approaches. Case studies of diverse FeIVO2+- and Cu(III)-based homogeneous and heterogeneous systems are presented to illustrate their practical applications in organic pollutant degradation and microbial inactivation, as well as the influence of water matrix components on treatment efficacy. The review concludes with a perspective on key research directions needed to advance HVM-based oxidation as a sustainable and effective strategy for water treatment.