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High temperature corrosion is the degradation of a material by chemical reaction with a hot gas, a molten salt or a deposit, at temperatures where the reaction product is a solid scale rather than an aqueous electrolyte film. In practice the dominant form is oxidation: a growing oxide layer separates the metal from the gas, and the rate is then controlled by the transport of ions or electrons through that layer, which is why the growth usually follows a parabolic law and why the composition and the defect structure of the scale matter more than the composition of the alloy. A protective scale is one that is dense, slow-growing and adherent, and the classical studies of refractory-metal oxidation established how quickly an alloy fails when the oxide is none of these ^[1]. The same criteria are applied to the compositionally concentrated alloys now being developed, where the oxidation behaviour of a medium-entropy alloy has been measured directly ^[2]. The results for the high-entropy families have been critically reviewed ^[3]. Surface treatments that introduce a second element into the surface layer improve the resistance by promoting a more protective scale ^[4]. Where an oxide is deliberately formed before service, as on zirconium alloy cladding, the initial scale changes the subsequent kinetics ^[5]. Spallation on thermal cycling, and attack by molten deposits, remain the limiting mechanisms.