Abstract In this chapter, we will explore several important questions about human mental abilities. In many ways, these questions are similar to those that we have addressed throughout this book in the context of personality variation. We will begin with the issue of the structure of mental abilities: Is there a general tendency for some people to be “smarter” than others across the full array of intellectual tasks? Or, is there a tendency for some people to be “smart” at some intellectual tasks and for other people to be “smart” at other such tasks? After discussing the structure of mental abilities, we will consider a series of questions about the nature of mental ability. How do levels of mental ability change throughout the life span, and how stable are individual differences in mental ability across long periods of time? What are the biological variables that underlie variation among people in mental ability? Is this variation mainly attributable to genetic or to environmental differences? To the extent that the environment is involved, which aspects have the strongest impact on mental abilities? We will then move on to the question of whether mental abilities have any important influences on real-life outcomes: Do scores on mental ability tests predict academic performance? Do they predict job performance or health and longevity? Being a good citizen? Whom we marry? Next, we will examine the sometimes complex ways in which mental abilities are related to several other variables. For example, some aspects of mental ability are substantially related to certain personality characteristics, whereas other aspects are not. Similarly, some aspects of mental ability show large differences between the generations and large changes throughout the life span, whereas other aspects do not. Finally, we will also consider some alternative ideas about mental ability that have achieved some popularity among the general public, such as the “triarchic theory of intelligence,” the “theory of multiple intelligences,” and “emotional intelligence.” In particular, we will examine the evidence regarding the validity of these ideas.