This chapter describes the question of what g is by addressing g as a psychometric score and examining its psychological and physiological correlates. The chapter discusses tacit knowledge and other non-g characteristics, and reviews the practical utility of g in personnel selection. Psychometrically, g can be extracted from a battery of tests with diverse content. The use of noncognitive traits, specific abilities, and knowledge has often been proposed as critical in personnel selection and for comprehension of the relations between human characteristics and occupational performance. The chapter shows the ubiquitous influence of g\ it is neither an artifact of factor analysis nor just academic ability. It considers criteria throughout the life cycle including educational achievement, training performance, job performance, lifetime productivity, and finally early mortality. None of this can be said for specific abilities. The chapter concludes by explaining its importance to both organizations and individuals.