The Blue Tit, Cyanistes caeruleus, is a 10.5 to 12 cm (4.2 to 4.8 inches,) long passerine bird in the tit family Paridae. It is a widespread and common resident breeder throughout temperate and subarctic Europe and western Asia in deciduous or mixed woodlands. It is a resident bird; i.e., most birds do not migrate.

The azure blue crown and dark blue line passing through the eye and encircling the white cheeks to the chin give the Blue Tit a very distinctive appearance. The forehead and a bar on the wing are white. The nape, wings and tail are blue; the back is yellowish green; the under parts mostly sulphur-yellow with a dark line down the abdomen. The bill is black, the legs bluish grey, and the irides dark brown. The sexes are similar, but under ultraviolet light, males have a brighter blue crown. Young Blue Tits are noticeably more yellow.

This is a common and popular European garden bird, due to its perky acrobatic performances when feeding on nuts or suet. It swings beneath the holder, calling "tee, tee, tee" or a scolding "churr".

The song period lasts almost all the year round, but is most often heard during February to June.

It will nest in any suitable hole in a tree, wall, or stump, or an artificial nest box, often competing with House Sparrows or Great Tits for the site. Few birds more readily accept the shelter of a nesting box; the same hole is returned to year after year, and when one pair dies another takes possession. It is estimated by the RSPB that there are 3,535,000 breeding pairs in the UK.

The bird is a close sitter, hissing and biting at an intruding finger. In the South West of England such behaviour has earned the Blue Tit the colloquial nickname "Little Billy Biter". When protecting its eggs it raises its crest, but this is a sign of excitement rather than anger, for it is also elevated during nuptial display. The nesting material is usually moss, wool, hair and feathers, and the eggs are laid in April or May. The number in the clutch is often very large, but seven or eight are normal, and bigger clutches are usually laid by two or even more hens. It is not unusual for a single bird to feed the chicks in the nest at a rate of one feed every ninety (90) seconds during the height of the breeding season. Its colourful mix of blue, yellow, white and green make the blue tit one of the most attractive resident garden birds. Almost any garden with a peanut feeder will attract them and they readily breed in nestboxes. In winter they form flocks with other tit species and a garden with four or five at a bird table at any one time, may be feeding 20 or more

Blue and Great Tits form mixed winter flocks, and the former are perhaps the better gymnasts in the slender twigs. A Blue Tit will often ascend a trunk in short jerky hops, imitating a Treecreeper. As a rule the bird roosts in ivy or evergreens, but in harsh winters will nest wherever there is a suitable small hole, be it in a tree or nesting box. Blue tits are very agile and can hang from almost anywhere.

The Blue Tit has an average life expectancy of 1.5 years.

The Blue Tit is a valuable destroyer of pests, though it has not an entirely clean sheet as a beneficial species. It is fond of young buds of various trees, and may pull them to bits in the hope of finding insects. No species, however, destroys more coccids and aphids, the worst foes of many plants. It takes leaf miner grubs and green tortrix moths (Tortricidae). Seeds are eaten, as with all this family.