The present paper reviewed the literature supporting and contradicting three theories of categorical perception in speech; the specialized speech processor theory, the cue duration theory, and the acoustic percept theory. Three experiments were then carried out to further investigate these theories. The first experiment compared subjects' perception of speech and three types of nonspeech stimuli (simple nonspeech, complex nonspeech, and reverse speech) and the effect of training on subjects' ability to discriminate these stimuli. The second experiment, dealt with a failure to replicate Pisoni (1977) using complex speech stimuli with the same parameters. The third experiment, compared speech and nonspeech perception in aphasic patients. Results failed to lend support to the specialized speech processing theory. Support for the cue duration theory and acoustic percept theory was mixed.