Among the most ancient vertebrates in the sea, ancestors of modern sharks originated almost 400 million years ago.

Today sharks live in essentially the same way they did more than 200 million years ago, before the rise of the dinosaurs. Scientists have identified nearly 375 species of sharks living today, ranging in size from the dwarf dogfish, less than 20 cm (8 in) in length, to the massive whale shark, which reaches lengths of more than 15 m (50 ft) . Most sharks inhabit tropic or temperate marine waters, but some species have been found in polar seas. The bull shark leaves ocean waters to enter freshwater rivers and lakes, including the Zambezi River in southeastern Africa, the Mississippi River in the United States, and Lake Nicaragua in southwestern Nicaragua. Depending on the species, sharks inhabit either shallow coastal waters or the open ocean. Some species, such as the six gill shark, live at depths of more than 1,800 m (6,000 ft).

Humans have hunted sharks for sport, food, medicine, and leather for centuries, with little regard for the health of shark populations. Sports fishers around the world regard sharks as some of the most challenging fish to catch in the sea. Shark flesh is highly prized in many regions of the world. One particularly popular food made from shark meat, shark fin soup, is in such demand that some fishers hunt sharks just for their fins, throwing the rest of the fish back to the sea to die. Shark liver oil is a popular source of vitamin A, and some people believe that shark liver and cartilage are beneficial to human health. Shark skin, with its microscopic teeth-like scales, was once used as a fine grade of sandpaper, and when the scales are removed from the skin to make shark leather, it brings high prices for use in shoes, belts, and handbags. Many sharks are killed unintentionally. Each year, thousands of sharks die in nets set out to catch other types of fish. Sometimes, humans kill sharks just because they fear them.

Such activities have placed many shark populations in danger of extinction. For example, between the mid-1980s and mid-1990s, populations of dusky sharks and sandbar sharks off the eastern coast of the United States declined by more than 80 percent. Internationally, the sand tiger shark and the great white shark are also in danger of extinction. Sharks grow slowly, reproduce late in their lives, and produce few offspring when they mate, making the natural rate of population replenishment very slow. If too many sharks in a particular area are killed, that population may never recover. For example, numbers of porbeagle sharks, swift, ocean-going sharks once commercially valuable, declined dramatically until, by 1960, commercial fishers could no longer catch enough of them to cover their expenses. Thirty years later, porbeagle populations still have not recovered.

As researchers begin to better understand the devastation human activities have brought to many shark populations, they also better understand the benefits sharks provide. Losing these top-level predators produces devastating effects to local ecosystems. Moreover, medical researchers are interested in learning more about sharks, particularly their immune systems. Sharks recover rapidly from severe injuries. They appear to be nearly impervious to infection, cancers, and circulatory diseases. For decades some people have believed that shark cartilage has anticancer properties. Although recent scientific studies challenge this belief, medical researchers continue to investigate the shark immune system in hopes of one day applying its secrets to fight human disease.

To prevent the effects of overfishing and other human activities in the United States, state and federal management plans restrict the number of sharks that can be legally killed. Plans also require that fishers take entire sharks, instead of just the fins, preventing the practice of cutting off the fins and leaving the rest of the fish to waste. In many countries, including South Africa, Australia, and the United States, legislation specifically protects great white sharks, a species widely prized as a game fish and considered to be endangered in many areas where they once roamed in large numbers.

While nets around bathing beaches prevent sharks from entering popular waters, such nets claim the lives of thousands of sharks each year. More sophisticated methods to repel sharks are under investigation. Chemical substances similar to fish toxins have been developed, and the repellent effects of many detergents may offer a chemical means to deter shark attacks. Electrical devices that interfere with shark sensory systems may one day offer another alternative to nets.