A critical survey is made of recent literature pertaining to the possibility of measuring important dimensions of human cognitive ability through various types of simple cognitive tasks. The survey focuses on individual differences (IDs) found in cognitive tasks that have received extensive study in experimental cognitive psychology--for example, simple and choice reaction time, lexical decision, stimulus matching, probed search of short-term memory, memory span, free and serial recall, simple sentence comprehension, and analogical reasoning. Questions of paramount interest were: What kinds of individual differences are observed in these tasks? Do they exhibit sufficient variance and reliability to measure stable individual characteristics that may be of relevance in personnel selection and other operational context? What are the dimensions of these IDs from a factor-analytic standpoint? How are they related to those observed through more conventional psychometric tests? To what extent are they subject to the effects of practice and specific training, or related to demographic variables in such a way as to reduce their utility for operational use? Can the study of individual difference in simple cognitive tasks be valuable in the development of psychological theory? A considerable portion of the monograph is devoted to the definition and specification of 'elementary cognitive tasks' (ECTs) and to how ETCs can be administered, in experimental settings, so as optimally to reveal IDs in cognitive processes. A scheme for classifying ECTs according to basic paradigms is presented, and suggestions are made concerning procedures, including computer simulation, for conceptualizing and analyzing underlying processes.