Penguins are a group of aquatic, flightless birds living almost exclusively in the Southern Hemisphere.

Depending on which authority is followed, the number of penguin species biodiversity varies between 17 and 20 living species, all in the subfamily Spheniscinae. Some sources consider the White-flippered Penguin a separate Eudyptula species, while others treat it as a subspecies of the Little Penguin (e.g. Williams, 1995; Davis & Renner, 2003);; the actual situation seems to be more complicated (Banks et al. 2002);. Similarly, it is still unclear whether the Royal Penguin is merely a color morph of the Macaroni penguin. Also possibly eligible to be treated as a separate species is the Northern population of Rockhopper penguins (Davis & Renner, 2003);. 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. At least ten species live in the temperate zone; one lives as far north as the Galápa;s Islands (the Galápa;s Penguin);.

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 (also known as the Fairy Penguin or the 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 or as heavy as an adult human.

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 predators such as the leopard seal.