The transition from kindergarten to first grade has been described as a critical period for children's academic development. Furthermore, research indicates that peer status is connected with academic adjustment, yet the underlying processes remain unclear. By means of a two-year longitudinal study during kindergarten and first grade (N = 153), we aimed to shed light on the antecedents of achievement at the end of first grade. Based on the parallel processes mediation model (Buhs, 2005), a comprehensive predictive model was constructed and tested. Results showed that (a) the parallel processes mediation model is partially valid during the transition from kindergarten to first grade; and (b) there is more support for an effect of academic self-concept on achievement than vice versa. This comprehensive model increases our insight in the factors that enhance children's academic development during the transition to first grade. (Contains 3 tables and 2 figures.)