The Emydids are the largest, most diverse family of living turtles, with about 100 species in Europe, Africa, and the Americas. Their use of habitat is equally diverse. A few are terrestrial, living on land, such as box turtles and wood turtles. Most are aquatic, living in ponds, streams, or lakes. However, new scientific techniques have revealed that many aquatic Emydids spend a lot of time on land. Turtles are secretive and well-camouflaged, so it is tough to find them. Recently, scientists began gluing small signal-emitting devices (radiotransmitters) to turtles and then followed them for months or years using an antennae to detect the signals. While it was already known that aquatic turtles nest on land, radio-tracking revealed that they may also hibernate for the winter on land, travel over land to other bodies of water with better food supplies, or even just warm up while buried under leaves in a sunny spot.

Like all turtles, Emydids reproduce sexually. Reproduction occurs seasonally, usually spring and summer, with males and females engaging in courtship followed by mating. Courtship is a prolonged affair, lasting as long as several hours. Emydid males often develop special features to engage with females. In some cooters, male toenails on the front feet grow extra long like curved toothpicks. The male uses them to stroke the female's head or just vibrates them alongside to titillate her. If she is receptive, she may stroke his legs. Once he is mounted in a mating position, he rakes the super-nails across her upper shell (carapace). During the breeding season, some Emydid males develop bright coloration on their necks, front limbs, or head. Whether these colors are used to display dominance to other males or to attract females is not known. Regardless, a male in breeding coloration is a showy sight.

The body temperature of a turtle changes with the temperature of its environment, making cold-blooded a misnomer. The blood temperature of a turtle sitting on a rock in full sunshine may be quite warm. Its dependence on outside temperatures, or ectothermy (ecto= outside; thermy= temperature) compels turtles to seek places with desirable temperatures. Every turtle species has a preferred temperature range for digestion and other metabolic processes. To get within range, some turtles need to sit in the sun (bask) for at least part of the day. Basking also dries out a turtle's shell, which might inhibit growth of algae or drive away other parasites that need moisture. A typical pattern is to bask in the morning until its body temperature gets high enough for a turtle to be active. Time spent seeking food (foraging) competes with basking. A turtle often alternates basking with foraging during the daylight hours.

Turtles do not parent their young. After the female lays eggs, she leaves. Each young turtle must break out of the egg using a caruncle, a sharp piece of keratin (fingernail material) on its jaw. Hatchlings may not get to feed right away, so they rely on take-out food. The yolk sac attached to their bellies, containing leftovers from the egg, sustains them. Still, when a hatchling leaves the nest, its odds of survival are poor. Its shell is still soft, making it edible to lots of predators, and outside conditions are tough. Sea turtles must reach the ocean without being attacked by crabs or shorebirds. Freshwater turtles must travel long distances, sometimes over steep terrain, dodging foxes and raccoons. Land turtles like tortoises risk getting dried out before they find water. Some hatchlings stay in the nest through an entire winter, living off the yolk sac and emerging when the weather warms.

The earliest archaeological evidence of humans eating turtles is at a site in northern Spain that is more than one million years old. In historic time, turtles remained popular as food. Huge lakes in the Central Valley of California were trawled by boats, who supplied the markets of San Francisco with thousands of turtles during the early 1900s. Australian aborigines today eat roasted turtle meat, using their shells as serving dishes. Amazonian tribes harvest river turtles for meat and fat. Farm-raised snapping turtles in the southeastern U.S. are the turtle in turtle soup. Asian cuisine particularly favors turtles. Massive turtle farming operations in China churn out soft-shell turtles for consumption, and turtle meat might grace the table at a traditional wedding. Besides their value as food, turtles are exploited heavily for the pet trade, and in Central America their eggs fetch a high price for their supposed aphrodisiac properties.