Use of an object by animals as a functional extension of their limbs in order to obtain food or to facilitate some other goal seeking activity has quite commonly been reported as an especially significant indicator of intelligence or complex learning ability. The present review has selected well authenticated examples of tool-using behavior from different types of animal, such as wasps, crabs, birds, subprimate mammals, and nonhuman primates, and examined the context of their occurrence and the apparent complexity of performance involved. These performances have been concerned with: a) attainment of food; b) offensive or defensive use against predators or intruders; c) miscellaneous functions such as self-grooming, courtship, nest-building. Categories (a) and (b) contain by far the most instances, and (c) has very few indeed. The problem, in attempting a comparative analysis of such instances, is to evaluate the performance within the whole context of the animal's capacities and the way these are expresse...