This chapter focuses on the types of human abilities and their correlates, although practical intelligence and tacit knowledge are reviewed. One of the remarkable features of human intelligence is its relative stability of individual differences over years, even decades. When longitudinal data are collected on the same person over time, it is possible to compute correlations of ability test scores across that interval. Older adults may also be effective at using strategies that enhance cognition in everyday life, such as through the use of external aids or behavioral routines that support timely remembering of what to do and when to do it. The study of adult cognitive and intellectual development is entering a vibrant new phase, one in which the advances in statistical methods for modeling individual differences are being integrated with designs and measures that permit a subtle understanding of individual differences in cognitive change.