It has long been documented that emotional and sensory events elicit a pupillary dilation. Is the pupil response a reliable marker of a visual detection event while viewing complex imagery? In two experiments where viewers were asked to report the presence of a visual target during rapid serial visual presentation (RSVP), pupil dilation was significantly associated with target detection. The amplitude of the dilation depended on the frequency of targets and the time of target presentation relative to the start of the trial. Larger dilations were associated with trials having fewer targets and with targets viewed earlier in the run. We found that dilation was influenced by, but not dependent on, the requirement of a button press. Interestingly, we also found that dilation occurred when viewers fixated a target but did not report seeing it. We will briefly discuss the role of noradrenaline in mediating these pupil behaviors.