Alvarezsauridae is a family of small, long-legged dinosaurs. Although originally thought to represent the earliest known flightless birds, they are now thought to be an early diverging branch of maniraptoran theropods. Alvarezsaurids were highly specialized, with tiny, stout forelimbs ending in compact, bird-like hands. Their skeletons suggest that they had massive breast and arm muscles, possibly adapted for digging or tearing. They had long, tube-shaped snouts filled with tiny teeth. They have generally been interpreted as myrmecophagous, adapted to prey on colonial insects such as termites, with the short arms acting as effective digging instruments to break into nests. Alternative hypotheses of ovivory interpret the manus as specialized shell-cracking apparatuses, using the large claws and robust forelimbs to puncture eggs.

Alvarezsaurus, the type genus of the family, was named after the historian and physician Gregorio Álvarez. Fossils attributed to alvarezsaurids have been found in North America, South America, and Asia, and range in age from about 86 to 66 million years ago.

Bonaparte (1991) described the first alvarezsaurid, Alvarezsaurus calvoi, from an incomplete skeleton found in Patagonia, Argentina. Bonaparte also named a family, Alvarezsauridae, to contain it. He argued that Alvarezsaurus might be most closely related to the ornithomimosaurs.

In 1993, Perle et al. described the next alvarezsaurid to be discovered, naming it Mononychus olecranus (meaning "one claw"). However, the spelling Mononychus was already in use by an extant beetle, so the name was changed to Mononykus a month later. Mononykus was initially described as a member of Avialae more advanced than Archaeopteryx, but the discovery of Patagonykus and reinterpretation of Alvarezsaurus created strong support for its inclusion in the Alvarezsauridae, and the subsequent inclusion of Alvarezsauridae within Avialae.

Several researchers considered it unlikely that alvarezsaurids were members of Avialae. Similarities between birds and alvarezsaurids—like the keeled sternum—were argued to be examples of homoplasy, where the derived alvarezsaurids developed birdlike characters through convergent evolution rather than inheriting them from a common ancestor with birds. Sereno noted in 1997 that alvarezsaurids lack critical bird features, and in 1999 completed a new anatomical analysis of Dinosauria and found that alvarezsaurids were more parsimoniously related to the Ornithomimosauria.

However, when Chiappe et al. described the cranial material of Shuvuuia deserti in 1998, they found Alvarezsauridae to be a sister taxon to birds following Perle et al. Further cladistic analysis by Chiappe et al. refuted the avian placement, but still maintained its close relationship as an immediate outgroup to Aves. The 2010 description of the Chinese alvarezsauroid Haplocheirus sollers did not support this analysis, as Haplocheirus possesses more basal characteristics in line with non-avialan Maniraptorans.

Alvarezsaurids ranged from 50 centimetres (20 in) to 2 metres (6.6 ft) in length, although some possible members may have been larger, including the dubious European alvarezsaurid Heptasteornis that may have reached 2.5 metres (8.2 ft) long.

At least one specimen of alvarezsaurid, from the species Shuvuuia deserti, preserved down-like, feathery integumental structures covering the fossil. Schweitzer et al. (1999) subjected these filaments to microscopic, morphological, mass spectrometric, and immunohistochemical studies and found that they consisted of beta-keratin, which is the primary protein in feathers.