Male uppersides are an iridescent lilac blue with a thin black border. Females are brown with a row of red spots along the edges. They usually have some blue at the base of the wings and, especially in Ireland and Scotland, are mostly blue but always have the red spots. Undersides have a greyish ground colour in the males and more brownish in the females. Both sexes have a row of red spots along the edge of the hindwings (extending onto the forewings though generally fainter, particularly in the males where they are sometimes missing altogether). There are about a dozen black centered white spots on the hind wings, nine on the forwings. The white fringe on the outer edge of the wings is not crossed with black lines as it is in the Chalkhill and Adonis Blues, an important difference when separating these species, particularly the females.
It is Britain's (and probably Europe's) most common and most widespread blue, found as far north as Orkney and on most of the Outer Hebrides. Males are often very obvious as they defend territories against rivals and search out the more reclusive females. A range of grassland habitats are used: meadows, coastal dunes, woodland clearings and also many man made habitats, anywhere where their food plants are found.
It is widespread in Europe, North Africa and temperate Asia.
The main food plant on most sites is Bird's foot trefoil (Lotus corniculatus). Others used include Black Medick Medicago lupulina, Common Restharrow Ononis repens, White Clover Trifolium repens and Lesser Trefoil Trifolium dubium. Eggs are laid singly on young shoots of their food plants.
The caterpillar is small, pale green with yellow stripes and as usual with lycid larvae rather slug-like. Hibernation occurs as a half grown larvae. They are attractive to ants but not as much as some other species of blues. The chrysalis is olive green/brown and formed on the ground where it is attended by ants which will often take it into their nests. The larvae creates a substance called honey dew, which the ants eat while the butterfly lives in the ant hill. In the south of Britain there are two broods a year flying in May and June and again in August and September. Northern England has one brood flying between June and September. In a long warm year there is sometimes a partial third brood in the south flying into October.