Scale insects are phytophagous, feeding by sucking plant juices through a set of highly modified mouthparts called stylets, and they are found on nearly every part of their hosts. They are widely distributed throughout the world with the exception of the cold extremes of the Arctic and Antarctic. Host plant diversity is broad, although scales are not commonly found on ferns or mosses. There is some debate about their rank in the classification system, but they are considered by many to be part of the Order Hemiptera, Suborder Sternorrhyncha, and Superfamily Coccoidea. The group includes nearly 8,000 species, 1,150 genera, and 32 extant families.

Scale insects are typically small, cryptic creatures, and many species cause major problems in agricultural and ornamental ecosystems. They are commonly transported on plant materials and because of their small size and habit of feeding in concealed areas, they are frequently transported outside their native range. If a species becomes established in its new environment, it may eventually become invasive. Invasive scales cause billions of dollars in damage annually.

Scales are characterized by having a single claw, neotenic adult females, winged but non-feeding adult males, and an unusual form of metamorphosis that normally includes a prepupa and pupa in the adult male. Generally there are 3 or 4 instars in the female and 5 instars in the male. Most scale insects produce some kind of wax covering, which can vary from a mealy or powdery substance covering the body to elaborate waxy structures that are attached to the body or formed as domicile-like structures.