One extension of Fechner’s (1860) methods by which the influence of sensory stimuli on human observers could be quantified in physical units, sometimes termed a psychophysics was Stevens (1938) theory and methodology of perceived magnitude estimation that attempted to bridge the gap between sensation and perception. A more far-reaching second extension began in 1954, when Peterson, Birdsall, and Fox applied statistical decision theory to the recovery of physical signals embedded in background noise and Tanner, Green, and Swets applied those principles to develop a new psychophysical methodology. What became known as “signal detection theory” provided methods of determining the capacity of systems to detect, discriminate, or recognize physical stimuli, a second psychophysics. The two alternatives to Fechner represent complimentary approaches to a single set of phenomena, one of which can be characterized as the study of sensory capabilities and the other of response proclivities. The two require entierly different methods of measurement and address fundamentally different questions. Both are of practical and theoretical importance. Levels of stimulus uncertainty and the type and duration of experience with stimuli can have profound influences on measurements of both types. Examples of these effects for complex sounds, both speech and nonspeech, will be presented.