This chapter concerns research and theory on the ability of nonhuman animals to learn, remember, and discriminate between events that differ in duration (e.g., a 2-second light vs. a 4-second light) and between those that differ in number (e.g., two light flashes vs. four light flashes)—in each case, events with temporal extension, one of the dimensions of complexity considered by Fetterman (1996). It begins with a history of attempts to understand how nonhuman animals discriminate temporal intervals and moves to a brief presentation of how animals discriminate numerosities. The remainder of the chapter is devoted to comparisons of duration and numerosity discriminations, and conclusions that may be drawn from the results of experiments on time and number discriminations. The main focus is on the role of stimulus information as opposed to underlying mechanisms.