Do nonhuman animals share humans' capacity for metacognition--that is, for monitoring or regulating their own cognitive states? Comparative psychologists have approached this question by testing a dolphin, pigeons, rats, monkeys and apes using perception, memory and food-concealment paradigms. There is growing evidence that animals share functional parallels with humans' conscious metacognition, although the field has not confirmed full experiential parallels and this remains an open question. This article reviews this new area of comparative inquiry and describes significant empirical milestones, remaining theoretical milestones and the prospects for continuing progress in a rapidly developing area. This research area opens a new window on reflective mind in animals, illuminating its phylogenetic emergence and allowing researchers to trace the antecedents of human consciousness.