The moments that follow a perceptual decision can be as critical as the ones leading up to it. This thesis examines rapid olfactory decisions in mice and rapid changes of mind following an initial decision in humans. To study olfactory decisions, we trained mice to discriminate different odor concentrations in a novel head-fixed paradigm. Odor concentration is an important cue for localizing odor sources, from searching for food and mates to avoiding predators. However, little is known about how olfactory systems encode odor concentration. We found that mice could discriminate the different odor concentrations in as little as 80 ms after odorant inhalation. This is an unprecedented speed for olfaction and is fast even by the standards of other sensory modalities. We attribute this new insight into olfactory perception to our choice of motor output, accurate sniff measurement, and precise stimulus control. This result suggests that odor-based decisions can be very rapid and based on information over a very brief temporal window. To study changes of mind following an initial decision, humans made decisions about a noisy visual stimulus, which they indicated by moving a handle. Although they received no additional information after initiating their movement, their hand trajectories betrayed a change of mind on some trials. We modeled these changes of mind by extending the diffusion-to-bound model, originally developed to account for both the timing and accuracy of the initial decision. We propose that noisy evidence is accumulated over time until it reaches a criterion, or bound which determines the initial decision, and that the brain exploits information that is in the processing pipeline when the initial decision is made to subsequently either reverse or reaffirm the initial decision. Sensory and motor processing latencies ensure that not all of the information available from stimulus onset to movement initiation contributes to the initial decision. This model explains both the frequency of changes of mind, as well as their dependence on both task difficulty and whether the initial decision was accurate or erroneous.