Grebes are members of the Podicipediformes, a widely distributed order of freshwater diving birds, some of which visit the sea when migrating and in winter. Grebes are small to medium-large in size, have lobed toes, and are excellent swimmers and divers. However, they have their feet placed far back on the body, making them quite ungainly on land. They leave the water only to nest, walking very short distances upright like penguins. They can run for a short distance, but often fall over. Grebes have narrow wings, and some species are reluctant to fly; indeed, two South American species are completely flightless. They respond to danger by diving rather than flying, and are in any case much less wary than ducks. However, the North American and Eurasian species are all, of necessity, migratory over much or all of their ranges, and those species that winter at sea are also seen regularly in flight. Even the small freshwater Pied-billed Grebe of North America has occurred as a transatlantic vagrant to Europe on more than 30 occasions.

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Bills vary from short and thick to long and pointed; the feet are always large, with broad lobes on the toes and small webs connecting the front three toes. The hind toe also has a small lobe. Recent experimental work has shown that these lobes work like the hydrofoil blades of a propeller. Curiously, the same mechanism seems to have evolved independently in the extinct Cretaceous-age Hesperornithiformes. Grebes have unusual plumage. It is dense and waterproof, and on the underside the feathers are at right-angles to the skin, sticking straight out to begin with and curling at the tip. By pressing their feathers against the body, grebes can adjust their buoyancy. Often, they swim low in the water with just the head and neck exposed.

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