Kinglets range in size from 9 to 11 cm and are amongst the smallest of the passerines, and indeed of all birds. They have medium-sized wings and tails, and the tails of all species are incised at the tip. The bill is small and needle-like. The plumage is overall grey-green. Most have an eye-ring or a stripe at the supercilium, although the slightly atypical Ruby-crowned Kinglet lacks this. The males possess a colourful crown patch. They have one specific feather which projects forward over the nares (again lacking in the Ruby-crowned Kinglet).
Generally kinglets are adapted to conifer forests, although there is a certain amount of adaptability and most species will use other habitats, particularly during migration. They have a distribution that takes in the northern boreal forests, but also extend down as far as the subtropics in places. A number of insular populations have evolved into separate species and subspecies.
The tiny size and rapid metabolism of kinglets means that they must constantly forage in order to provide their energy needs. They will continue feeding even when nest building. Kinglets prevented from feeding may lose a third of their body weight in twenty minutes and may starve to death in an hour. Kinglets are insectivores, preferentially feeding on insects such as aphids and springtails that have soft cuticles. Prey is generally gleaned from the branches and leaves of trees, although in some circumstances prey may be taken on the wing or from the leaf litter on the ground.
The nest are small, very neat cups, almost spherical in shape, made of moss and lichen held together with spiderwebs and hung from twigs near the end of a high branch of a conifer. They are lined with hair and feathers, and a few feathers are placed over the opening. These characteristics provide good insulation against the cold environment. The female lays 7 to 12 eggs, which are white or pale buff, some having fine dark brown spots. Because the nest is small, they are stacked in layers. The female incubates; she pushes her legs (which are well supplied with blood vessels, hence warm) down among the eggs. The eggs hatch after 15 to 17 days. The young stay in the nest for 19 to 24 days. After being fed, nestlings make their way down to the bottom of the nest, pushing their still-hungry siblings up to be fed in their turn (but also to be cold).