Examining how students' epistemic beliefs (EB) influence their cognition is central to EB research. Recently, the relation between students' EB and their motivation has gained attention. In the present study, we investigate the development of the relationship between students' EB and their achievement goals (AG) over grades 5-11. Previous studies on this topic are limited, in both number and range, and have produced inconsistent results. We performed a cross-sectional study, ranging over grades 5-11, and a 3-year longitudinal study (n = 1230 and 323, respectively). Data on students' EB and AG were collected via questionnaires. Confirmatory factor analysis (CFA) supported a two-factor goal model (Mastery and Performance goals) and a structure of students' EB comprising Certainty, Source, Development, and Justification. For each grade, students' CFA scores on the respective goals were regressed on their scores on the EB dimensions by orthogonal projection to latent structures analysis. Although results indicated a weak relation between students' EB and AG, trends in the cross-sectional data were largely replicated in the longitudinal study. Though naïve EB were in general associated with performance goals and sophisticated EB with mastery goals, the transition to upper secondary school was associated with changes in the relationship between students' EB and AG. We discuss how the commonly used formulations of EB items may affect their ability to measure the naïve-sophisticated continuum, in turn affecting the predictive roles of EB dimensions.