A by-product is a secondary or incidental product deriving from a manufacturing process or chemical reaction, and is not the primary product or service being produced. A by-product can be useful and marketable, or it can have severe ecological consequences.

Animal by-products often include parts of animals left over after butchering the animals for human consumption, such as feet, ears, heart, brains, and so on. Such by-products have often been used as sources of protein in food for animals, including dog food, cat food, and fodder for herbivorous livestock. The latter practice is evidently the primary cause of several international outbreaks of mad cow disease, which can be spread through the meat and bone meal created by grinding up leftover nerve tissue, including brains.