Waders or shorebirds are birds of the order Charadriiformes commonly found wading along shorelines and mudflats in order to forage for food crawling or burrowing in the mud and sand, usually small arthropods such as aquatic insects or crustaceans. The term "wader" is used in Europe, while "shorebird" is used in North America, where "wader" may be used instead to refer to long-legged wading birds such as storks and herons.

There are about 210 species of wader, most of which live in wetland or coastal environments. Many species of Arctic and temperate regions are strongly migratory, but tropical birds are often resident, or move only in response to rainfall patterns. Some of the Arctic species, such as the little stint, are amongst the longest distance migrants, spending the non-breeding season in the Southern Hemisphere.

Many of the smaller species found in coastal habitats, particularly but not exclusively the calidrids, are often named as "sandpipers", but this term does not have a strict meaning, since the upland sandpiper is a grassland species.

The smallest member of this group is the least sandpiper, small adults of which can weigh as little as 15.5 grams (0.55 oz) and measure just over 13 centimetres (5 inches). The largest species is believed to be the Far Eastern curlew, at about 63 cm (25 in) and 860 grams (1 pound 14 ounces), although the beach thick-knee is the heaviest at about 1 kg (2 lb 3 oz).

In the Sibley-Ahlquist taxonomy, waders and many other groups are subsumed into a greatly enlarged order Ciconiiformes. However, the classification of the Charadriiformes is one of the weakest points of the Sibley-Ahlquist taxonomy, as DNA–DNA hybridization has turned out to be incapable of properly resolving the interrelationships of the group. Formerly, the waders were united in a single suborder Charadrii, but this has turned out to be a "wastebasket taxon", uniting no fewer than four charadriiform lineages in a paraphyletic assemblage. However, it indicated that the plains wanderer actually belonged into one of them. Following recent studies (Ericson et al., 2003; Paton et al., 2003; Thomas et al., 2004a, b; van Tuinen et al., 2004; Paton & Baker, 2006), the waders may be more accurately subdivided as follows, with Charadrii being repurposed into a monophyletic suborder of plovers, oystercatchers, and their close relatives.

The waders are traditionally a group of two charadriiform suborders which include 13 families. Species in the third charadriiform suborder, Lari, are not universally considered as waders, though the families in Lari which may variously be included are listed below as well.

Shorebirds is a blanket term used to refer to multiple bird species that live in wet, coastal environments. Because most these species spend much of their time near bodies of water, many have long legs suitable for wading (hence the name 'Waders'). Some species prefer locations with rocks or mud. Many shorebirds display migratory patterns and often migrate before breeding season. These behaviors explain the long wing lengths observed in species, and can also account for the efficient metabolisms that give the birds energy during long migrations.

The majority of species eat small invertebrates picked out of mud or exposed soil. Different lengths of bills enable different species to feed in the same habitat, particularly on the coast, without direct competition for food. Many waders have sensitive nerve endings at the end of their bills which enable them to detect prey items hidden in mud or soft soil. Some larger species, particularly those adapted to drier habitats will take larger prey including insects and small reptiles.