The blue tit is usually 12 cm long with a wingspan of 18 cm for both sexes, and weighs about 11 grams. A typical blue tit has an blue crown and line passing through the eye, white cheeks to the chin, giving the bird a very distinctive appearance. The forehead and a bar on the wing are white. The nape, wings and tail are blue and the back is yellowish green. The underparts are mostly sulphur-yellow with a dark line down the abdomen—the yellowness is indicative of the number of yellowy-green caterpillars eaten, due to high levels of carotene pigments in the diet. The bill is black, the legs bluish grey.

They usually nest in tree holes, although they easily adapt to nest boxes where necessary.

The blue tit prefers insects and spiders for its diet. Outside the breeding season, they also eat seeds and other vegetable-based foods. The birds are famed for their acrobatic skills, as they can hold on to the outermost branches of trees and shrubs and hang upside down when looking for food.