This article has two purposes. The first is to describe four theoretical models of yes-no recognition memory and present their associated measures of discrimination and response bias. These models are then applied to a set of data from normal subjects to determine which pairs of discrimination and bias indices show independence between discrimination and bias. The following models demonstrated independence: a two-high-threshold model, a signal detection model with normal distributions using d' and C (rather than beta), and a signal detection model with logistic distributions and a bias measure analogous to C. C is defined as the distance of criterion from the intersection of the two underlying distributions. The second purpose is to use the indices from the acceptable models to characterize recognition memory deficits in dementia and amnesia. Young normal subjects, Alzheimer's disease patients, and parkinsonian dementia patients were tested with picture recognition tasks with repeated study-test trials. Huntington's disease patients, mixed etiology amnesics, and age-matched normals were tested by Butters, Wolfe, Martone, Granholm, and Cermak (1985) using the same paradigm with word stimuli. Demented and amnesic patients produced distinctly different patterns of abnormal memory performance. Both groups of demented patients showed poor discrimination and abnormally liberal response bias for words (Huntington's disease) and pictures (Alzheimer's disease and parkinsonian dementia), whereas the amnesic patients showed the worst discrimination but normal response bias for words. Although both signal detection theory and two-high-threshold discrimination parameters showed identical results, the bias measure from the two-high-threshold model was more sensitive to change than the bias measure (C) from signal detection theory. Three major points are emphasized. First, any index of recognition memory performance assumes an underlying model. Second, even acceptable models can lead to different conclusions about patterns of learning and forgetting. Third, efforts to characterize and ameliorate abnormal memory should address both discrimination and bias deficits.