The geometer moths or Geometridae (from Greek geo γη or γαια 'the earth' and metron μέτρων 'measure' — refers to the larvae, which appear to "measure the earth" as they move in a looping fashion) are a family of the order Lepidoptera. A very large family, it has around 35,000 species of moths described, and over 1,400 species from 6 subclasses indigenous to North America. A well-known member is the Peppered Moth, Biston betularia, which has been subject of numerous studies in population genetics. Several other geometer moths are notorious pests.

Many geometrids have slender abdomens and broad wings which are usually held flat with the hindwings visible. As such they appear rather butterfly-like but in most respects they are typical moths: the majority fly at night, they possess a frenulum to link the wings and the antennae of the males are often feathered. They tend to blend in to the background, often with intricate, wavy patterns on their wings. In some species, females have reduced wings (e.g. winter moth and fall cankerworm).