Baleen is a filter-feeding system inside the mouths of baleen whales. To use baleen, the whale first opens its mouth underwater to take in water. The whale then pushes the water out, and animals such as krill are filtered by the baleen and remain as a food source for the whale. Baleen is similar to bristles and consists of keratin, the same substance found in human fingernails, skin and hair. Baleen is a skin derivative. Some whales, such as the bowhead whale, have longer baleen than others. Other whales, such as the gray whale, only use one side of their baleen. These baleen bristles are arranged in plates across the upper jaw of whales.

Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Filter feeders can be sessile, planktonic or nektonic, and some extant animals that rely on this method of feeding are clams, krill, sponges, baleen whales and many fish. Some birds, such as flamingos and certain species of duck, are also filter feeders. Filter feeders can play an important role in cleaning water, and are therefore considered ecosystem engineers. They are also important in bioaccumulation and, as a result, as indicator organisms.