Although our subjective experience of the world is one of discrete sound sources, the individual frequency components that make up these separate sources are spread across the frequency spectrum. Listeners. use various simple cues, including common onset time and harmonicity, to help them achieve this perceptual separation. Our ability to use harmonicity to segregate two simultaneous sound sources is constrained by the frequency resolution of the auditory system, and is much more effective for low-numbered, resolved harmonics than for higher-numbered, unresolved ones. Our ability to use interaural time-differences (ITDs) in perceptual segregation poses a paradox. Although ITDs are the dominant cue for the localization of complex sounds, listeners cannot use ITDs alone to segregate the speech of a single talker from similar simultaneous sounds. Listeners are, however, very good at using ITD to track a particular sound source across time. This difference might reflect two different levels of auditory processing, indicating that listeners attend to grouped auditory objects rather than to those frequencies that share a common ITD.