This paper explores the power of personality traits as predictors and causes of academic and economic success, health, and criminal activity. Personality traits are interpreted as constructs derived from an economic model of preferences, constraints, and information. Evidence is reviewed about the "situational specificity" of personality traits and preferences. The paper examines the biological and evolutionary origins of personality traits, personality measurement systems, and the relationships among the measures used by psychologists. The predictive power of personality measures is compared with the predictive power of measures of cognition captured by IQ and achievement tests. For many outcomes, personality measures are just as predictive as cognitive measures, even after controlling for family background and cognition. Moreover, standard measures of cognition are heavily influenced by personality traits and incentives. Measured personality traits are positively correlated over the life cycle. However, they are not fixed and can be altered by experience and investment. Intervention studies, along with studies in biology and neuroscience, establish a causal basis for the observed effect of personality traits on economic and social outcomes. Personality traits are more malleable over the life cycle compared to cognition, which becomes highly rank stable around age 10. Interventions that change personality are promising avenues for addressing poverty and disadvantage. The paper discusses how personality psychology informs economics and how economics can inform personality psychology. It presents a definition of personality as a strategy function for responding to life situations. Personality traits, along with other influences, produce measured personality as the output of personality strategy functions. The paper examines the strengths and limitations of different approaches to defining traits. It links the abstract definition to linear factor models commonly used to identify personality and cognitive traits. The paper presents the main systems used to measure personality and cognition and discusses the relationship among the systems. It illustrates a nonidentification result by showing how scores on IQ tests are greatly affected by incentives and context. It presents additional evidence showing that scores on achievement tests depend on cognitive and personality measurements, with a substantial predictive role for personality measures. Measures of "IQ" commonly used in economics and social science conflate measures of cognition and personality. The paper discusses economic preferences and examines the evidence relating economic preference parameters to psychological parameters. It surveys the evidence on the predictive validity of personality measures for education, crime, health, and labor market outcomes. The material presented in the main text summarizes a large and growing empirical literature. A Web Appendix presents additional detail on the literature relating cognition and personality in each of these areas of economic and social life. The paper presents evidence on the causal impact of personality on outcomes, as well as evidence on the stability and malleability of personality traits and preferences. It extends the theoretical framework for trait formation introduced in Section 3 and discusses a corresponding measurement system. It discusses the evidence from intervention studies. The paper concludes with provisional answers to the eight questions posed.This paper explores the power of personality traits as predictors and causes of academic and economic success, health, and criminal activity. Personality traits are interpreted as constructs derived from an economic model of preferences, constraints, and information. Evidence is reviewed about the "situational specificity" of personality traits and preferences. The paper examines the biological and evolutionary origins of personality traits, personality measurement systems, and the relationships among the measures used by psychologists. The predictive power of personality measures is compared with the predictive power of measures of cognition captured by IQ and achievement tests. For many outcomes, personality measures are just as predictive as cognitive measures, even after controlling for family background and cognition. Moreover, standard measures of cognition are heavily influenced by personality traits and incentives. Measured personality traits are positively correlated over the life cycle. However, they are not fixed and can be altered by experience and investment. Intervention studies, along with studies in biology and neuroscience, establish a causal basis for the observed effect of personality traits on economic and social outcomes. Personality traits are more malleable over the life cycle compared to cognition, which becomes highly rank stable around age 10. Interventions that change personality are promising avenues for addressing poverty and disadvantage. The paper discusses how personality psychology informs economics and how economics can inform personality psychology. It presents a definition of personality as a strategy function for responding to life situations. Personality traits, along with other influences, produce measured personality as the output of personality strategy functions. The paper examines the strengths and limitations of different approaches to defining traits. It links the abstract definition to linear factor models commonly used to identify personality and cognitive traits. The paper presents the main systems used to measure personality and cognition and discusses the relationship among the systems. It illustrates a nonidentification result by showing how scores on IQ tests are greatly affected by incentives and context. It presents additional evidence showing that scores on achievement tests depend on cognitive and personality measurements, with a substantial predictive role for personality measures. Measures of "IQ" commonly used in economics and social science conflate measures of cognition and personality. The paper discusses economic preferences and examines the evidence relating economic preference parameters to psychological parameters. It surveys the evidence on the predictive validity of personality measures for education, crime, health, and labor market outcomes. The material presented in the main text summarizes a large and growing empirical literature. A Web Appendix presents additional detail on the literature relating cognition and personality in each of these areas of economic and social life. The paper presents evidence on the causal impact of personality on outcomes, as well as evidence on the stability and malleability of personality traits and preferences. It extends the theoretical framework for trait formation introduced in Section 3 and discusses a corresponding measurement system. It discusses the evidence from intervention studies. The paper concludes with provisional answers to the eight questions posed.