The aim of the study was to identify possible individual differences effects on school mathematics performance and feelings of difficulty (FOD). Cognitive ability (general and domain-specific), affect (anxiety and achievement need), age, and gender were considered the main sources of individual differences. The effect of the testing experience (i.e., the repeated exposure to the testing situation) was also taken into account. Two hundred forty three subjects, of both genders, from 13 to 15 years of age were tested with three task batteries: the cognitive ability, the affective battery and the school mathematics battery. Ratings of the difficulty of each of the mathematics items were also collected. A second testing of the affective battery, mathematical tasks and FOD was effected one year after the first. A series ofpath analyses and ANOVAs were performed on the data. It was found that ability directly influenced performance whereas both ability and affect influenced FOD. Feelings of difficulty were also influenced by performance. Age differentiated FOD only at the 2nd testing. Gender interacted with both person and task characteristics and had an effect on FOD but not on performance.As indicated in the title, this paper deals with individual differences in school mathematics. It summarises the main findings of a research project which was reported in Etklides, Kiosseoglou (1997, 1998). The distinct feature of this project was the emphasis it placed not only on performance but on subjective experience, in this case feelings of difficulty (FOD) experienced in relation to the mathematical tasks solved. From this point of view, our project aimed to show the relevance of individual differences (ID) research to the study of online experience and self-regulation, as mediated by cognitive and affective factors.