The theory of signal detection has provided refined measures of the capacity of human observers to detect sensory stimuli. But in addition, by incorporating aspects of decision making, the theory provides many connections between psychophysics and other areas of psychology. Advances in psychophysical methodology are no longer restricted in their usefulness to the study of sensory thresholds, but instead may provide powerful techniques for studying how people make decisions about environmental events in general. For example, TSD has been useful in studying decision processes ranging from how clinicians diagnose illness from a set of symptoms to how people decide whether or not they have seen a person’s face before. The research described below is but a small sample of the many applications of TSD to problems of determining how people make decisions.