Mental chronometry, in which conclusions about human information processing are reached through measures of subjects' reaction time, has contributed substantially to studies of cognition and action. During the evolution of the chronometric paradigm, several key issues have emerged. The issues concern (a) the existence of separable processing stages, (b) the degree to which various stages of processing produce partial outputs before they are completed, and (c) the discrete versus continuous form of the outputs. To obtain added temporal resolution, new reaction-time procedures have been developed, including special response-priming and speed-accuracy decomposition techniques that focus on quantitative patterns of reaction-time distributions and error rates. The present article summarizes these developments, starting with a historical review of chronometric research and proceeding to a survey of recent empirical and theoretical innovations. We also discuss the relevance and potential future impact of complementary work by cognitive psychophysiologists on event-related brain potentials and other physiological variables.