The aim of the present doctoral thesis was to examine to what extent personality traits and approaches to learning contribute to academic performance in upper secondary school (high school), after controlling for the well-known fact that general intelligence accounts for a large part of the variance. The general proposition of the thesis is that personality traits are stable dispositions and therefore predispose an individual to behave or act in a specific manner (Costa & McCrae, 1976). Additionally, another important determinant of academic performance is students’ approaches to learning, the way someone studies and makes sense of a particular school subject (Biggs, 1999). Study I examined how personality traits, divided into facets, predict academic performance in different school subjects. The results from several SEM analyses showed that personality, specifically Conscientiousness, has a positive influence on academic performance. In addition, there was a negative relation between Extraversion and academic performance and a positive relation between Neuroticism and academic performance. There were also interesting findings on the facet levels for all traits. The major conclusion of this study is that personality traits, both on the factor level and on the facet level, are important to academic performance in general, but sometimes more specifically to different school subjects. In Study II, the aim was to investigate the unique contribution of learning approaches to academic performance. A second aim was to explore possible gender differences in learning approaches. It was found that learning approaches contributed uniquely to academic performance, over and above personality and general intelligence. Differences between girls and boys were found, both with respect to the use of learning approaches and the consequences of these learning approaches for performance results. Based on a longitudinal design, the aim of Study III was to explore to what extent personality traits predict academic performance. Conscientiousness, Extraversion and Neuroticism were found to predict overall academic performance. Results suggest that personality traits, as measured at the age of 16, can predict academic performance at the age of 19, and more specifically: the grades of conscientious students improved from age 16 to age 19. This study extends previous work by assessing the relationship between the Big Five and academic performance over a three-year period.