Two approaches to the study of animal timing have emerged quite independently of each other. They both use the metaphor of the internal clock; they both are concerned with biological and experiential determinants of timing; and they both involve mathematical models of the process. One of these approaches is primarily concerned with periodic timing and the other with interval timing. The properties of the internal clock that are assumed, the variables affecting timing, and the nature of the mathematical models of the process are quite different. Periodic timing has been investigated mostly by biologists whereas interval timing has been investigated mostly by experimental psychologists. Not many investigators of one type of timing are familiar with the detailed results of the other type of timing, and the amount of cross-referencing to the recent empirical literature is limited.