Although all penguin species are native to the southern hemisphere, they are not, contrary to popular belief, found only in cold climates, such as Antarctica. In fact, only a few species of penguin actually live so far south. Three species live in the tropics; one lives as far north as the Galápagos Islands.

The largest living species is the Emperor Penguin: adults average about 1.1 m (3 ft 7 in) tall and weigh 35 kg (75 lb) or more. The smallest penguin species is the Little Blue Penguin, which stands around 40 cm tall (16 in) and weighs 1 kg (2.2 lb). Generally larger penguins retain heat better, and thus inhabit colder regions, while smaller penguins are found in temperate or even tropical climates. Some prehistoric species attained enormous sizes, becoming as high as an adult human; see below for more.

Most penguins feed on krill, fish, squid, and other forms of sealife caught while swimming underwater. They spend half of their life on land and half in the oceans.

When mothers lose a chick, they sometimes attempt to steal another mother's chick, usually unsuccessfully as other females in the vicinity assist the defending mother in keeping her chick.

Penguins seem to have no fear of humans and have approached groups of explorers without hesitation. This is probably on account of there being no land predators in Antarctica or the nearby offshore islands that prey on or attack penguins. Instead, penguins are at risk at sea from such predators as the leopard seal.