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The thesis is that the same timing mechanism is available and is used by experienced subjects in a synchronization task whether the intervals involved are relatively long (eg 2000 msec) or short (eg 200 msec) and whether the stimuli are auditory or visual. Synchronization involves two main stimuli. One marks the start of a target interval. The second marks its end. The subject is to respond in synchrony with the second stimulus. Previous synchronization research using short auditorily bounded intervals has strongly supported the hypothesis that the underlying mechanism is continuously adjustable, deterministic, and, despite continuous adjustability, that the main source of variability in the task is quantal. A periodic process in the nervous system controls the timing of the information transfer from one processing stage to the next. Waiting state durations (before transfers) are uniformly distributed from 0 to q msec, where q is the quantum size. As an example of a timing model along these lines, suppose there is a quantal delay between sensory registration of a stimulus marking the start of an interval and transfer of the message that the interval has started to a "perfect" central clock. In synchronization, the subject waits a specified time after the stimulus, then makes a response. The message from the clock that the required interval has elapsed is subject to another quantal delay before arriving at the motor system, which carries out the response. Variability at the sensory and motor levels may add to the total timing variance in the system, but the main source of variation in extremely experienced subjects is quantal. Data from various paradigms suggest that different mechanisms govern timing of long and short intervals and that different types of, or at least differentially variable, mechanisms are involved in timing visually and auditorily marked intervals. Both hypotheses have received some support from past synchronization research though they have received much less support from duration discrimination studies that also suggest quantal mechanisms. There were five experiments, two involving long auditory intervals, one with short auditory intervals, and two with short visual intervals. All aspects of the thesis were supported: similar practice effects and distributional shapes are found for short auditory and visual intervals. Performance at longer intervals differs from these in variance only, which is concordant with discrimination results, though a specific quantal counting model for discrimination (Kristofferson, 1980) that has had difficulty with previous synchronization data (Hopkins, 1982) has the same difficulty with the present results. The data support the hypothesis of a common centraI timing mechanism across modalities and intervals to 2000 msec, and also the idea that boundary conditions on quantal theory found applicable in any given paradigm and modality will also apply to the others. Doctor of Philosophy (PhD)