This volume is based on the 10th annual Harvard Symposium for the Quantitative Analysis of Behavior. The first Harvard Symposium was devoted to signal-detection analyses of reinforcement and choice behavior. The present volume reprises the original signal- detection theme, incorporating additional insights based on experimental and theoretical analyses undertaken during the years separating the two conferences. This collection illustrates how signal-detection theory, first advanced to account for performance in threshold-level sensory discrimination, has broadened to encompass a variety of psychological problems involving discriminations between confusable stimuli. The approach is quantitative in its emphasis on estimation of independent parameters of the discrimination process, and analytical in its efforts to separate the determiners of discriminability and bias and to identify the mechanisms of their operation. Above all, the book is broadly integrative in its approach to diverse problems. This volume is based on the 10th annual Harvard Symposium for the Quantitative Analysis of Behavior. The first Harvard Symposium was devoted to signal-detection analyses of reinforcement and choice behavior. The present volume reprises the original signal- detection theme, incorporating additional insights based on experimental and theoretical analyses undertaken during the years separating the two conferences.