The authors report 9 new experiments and reanalyze 3 published experiments that investigate factors affecting the time course of perceptual processing and its effects on subsequent decision making. Stimuli in letter-discrimination and brightness-discrimination tasks were degraded with static and dynamic noise. The onset and the time course of decision making were quantified by fitting the data with the diffusion model. Dynamic noise and, to a lesser extent, static noise, produced large shifts in the leading edge of the response-time distribution in letter discrimination but had little effect in brightness discrimination. The authors interpret these shifts as changes in the onset of decision making. The different pattern of shifts in letter discrimination and brightness discrimination implies that decision making in the 2 tasks was affected differently by noise. The changes in response-time distributions found with letter stimuli are inconsistent with the hypothesis that noise increases response times to letter stimuli simply by reducing the rate at which evidence accumulates in the decision process. Instead, they imply that noise also delays the time at which evidence accumulation begins. The delay is shown not to be the result of strategic processes or the result of using different stimuli in different tasks. The results imply, rather, that the onset of evidence accumulation in the decision process is time-locked to the perceptual encoding of the stimulus features needed to do the task. Two mechanisms that could produce this time-locking are described.