The holly blue (Celastrina argiolus) is a small lycaenid butterfly with violet-blue upperwings (males narrow-bordered, females broad-bordered) and pale silver-blue underwings spotted with black, wingspan 2.6–3.4 cm, widespread across temperate Eurasia from Britain and western Europe east to Japan, with additional populations in North Africa. It is notable for a two-host life cycle: the spring brood lays on holly (Ilex aquifolium) flower buds and berries, while the summer brood lays on ivy (Hedera helix) and other shrubs including spindle and dogwood. Adults show a marked preference for aphid honeydew over nectar, though both sexes will also visit flowers of holly, ivy, bramble, and forget-me-not. It is a common garden butterfly wherever holly or ivy grow, but populations fluctuate in multi-year cycles driven by the specialist parasitoid wasp Listrodromus nycthemerus.

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Found throughout temperate Eurasia from Britain and western Europe east to Japan, with additional populations in North Africa (subspecies mauretanica and algirica). In Great Britain it is most abundant south of a line from Cumberland to County Durham; scarce vagrant in Scotland. It is Finland's national butterfly. Note: some older taxonomic treatments included North American Celastrina populations (the spring azure complex) as subspecies of C. argiolus, but most current North American authorities (including NatureServe and Pelham) treat these as separate species, principally Celastrina ladon. IUCN status: Least Concern.