This chapter describes measures of cognitive ability (general mental ability and specific abilities) and examines their usefulness for personnel selection. An overview of definitional and theoretical issues as they apply to use of such measures in personnel decision making is provided first. Then, issues of reliability of measures are discussed, again with particular emphasis on implications for personnel selection (e.g., impact on rank order of candidates when using different measures). Next, validities of cognitive ability tests are summarized for the following criteria: overall job performance, task performance, contextual performance, counterproductive work behaviors, leadership, creativity and innovation, voluntary turnover, job satisfaction, and career success. The authors address the nature of predictor-criterion relationships (e.g., usefulness of general versus specific abilities, criterion dynamicity, assumption of linearity) by discussing both recent large-scale evidence in normal samples and among the highly gifted. Finally, the extent to which cognitive ability is captured in tools other than standardized tests is summarized, enabling an evaluation of other selection assessments as substitutes and/or supplements to standardized cognitive ability tests.