Metacognition is knowledge about knowledge, often expressed as confidence judgments about what we know. Most of the literature on metacognition in humans is based on subjects' verbal reports. Investigators of animal cognition have recently described nonverbal methods for investigating metacognition in animals. In one, subjects are given the option to escape from difficult trials. In another, subjects are trained to place bets about the accuracy of their most recent response. To rule out noncognitive interpretations of purported evidence of metacognition in animals, one must ensure that escape responses do not increase the overall density of reinforcement and that they do not occur in the presence of the stimuli on which the subject was trained. The nonverbal techniques used to investigate metacognition in animals make possible two interesting lines of research: investigating the contribution of language and explicit instruction in establishing metacognition, and the investigation of the neural substrates of metacognition.