Tool use was once considered as unique ability for humans. However, in the past 50 years, tool use has been widely observed throughout the animal kingdom, and therefore is more common than previously thought. Tool use in animals usually has certain purposes, and most animals use tools to achieve short-term goals instead of long-term goals. The tool-use behaviors may be influenced by environmental, cognitive, and physiological factors, and individual differences in tool use may be observed. For some animals, the tool use is an innate behavior. However, for most higher vertebrates, tool use usually involves the trial-and-error learning. Through social learning, some tool-use behaviors can spread and evolve, thus being widely distributed in the population. Tool use is one of the central concepts in animal cognition. Researches in tool use would enhance the understanding of cognitive abilities and behavioral evolution of animals.