The answer to the question of what whales eat is that it depends on the type of whale. Whales are divided into two suborders: baleen and toothed whales, and these whales eat very different kinds of food. Some studies have also suggested that human activities may have influenced whale feeding habits and the diet of whales, judging from reviews of historical evidence and studies of modern whales.

Toothed whales such as killer whales and pilot whales are equipped with a large set of teeth, as their name implies. These whales eat schooling fishes, squid, and marine mammals such as seals. Some species eat other whales and dolphins. In the case of killer whales, also known as orcas, the animals often work cooperatively to bring down prey much larger than themselves. Researchers have even documented instances of killer whales going after blue whales, the largest mammals on Earth.

Baleen whales are filter feeders which eat things like plankton, copepods, small fish, and krill. They filter their food through a comb-like structure known as baleen. Blue whales, minke whales, and right whales are some examples of baleen whales. Gray whales, another type of baleen whale, filter the sediment on the ocean floor to access the small creatures which live there. In the case of the blue whale, the whales eat up to 8,000 pounds (3,600 kilograms) of food every day during the season when they feed to store up energy reserves for the winter. In the winter months, the animals survive on their blubber.

The stored energy in the form of blubber is common to many marine mammals. In addition to being a source of energy when food is difficult to obtain, the blubber also provides insulation which protects the whales from extremely cold water, allowing them to travel to extreme latitudes. Blubber has also been of historical interest to humans as a source of fuel; whaling was once a worldwide industry which generated tremendous profits, primarily from the sale of blubber.

Studies to determine what whales eat can be conducted in a number of ways. Historically, anecdotal evidence from whalers was used to learn more about the diet of whales. In the modern era, researchers can sift through whale excrement to extract mitochondrial DNA which reveals the identity of the organisms the whale ate. This method has been promoted by researchers who do not want to kill whales to learn more about them.