This chapter discusses the auditory cognition by considering how speech is understood. An elementary presentation of certain phenomena in acoustics and linguistics is followed by an examination of the cognitive units involved. Distinctive features, phonemes, words, and linguistic constituents are all considered. The hypothesis that distinctive features are the critical stimuli in speech perception is attractive, but it has complexities of its own. The temporal complexity of speech perception, the varying sizes of units employed in different situations, the coexistence of brief distinctive features, longer syllabic rhythms, and still longer intonation patterns all argue against any discrete quantum of psychological time. In addition, it takes the motor theory perceptibly closer to a more flexible model of speech perception. The scale of the segment which is recognized or constructed varies at least from the syllable to the phrase, with a flexibility comparable to that found in vision.