We investigated the impact of self-assessment training on student achievement and on computer self-efficacy in a technology-supported learning environment (grade 9 students using Global Information Systems software). We found that self-assessment had a positive effect on student achievement, accounting for 25% of the variance across three measures. The treatment effect was as large for females as for males and for those with low initial self-efficacy as it was for those with higher scores. In addition, self-efficacy increased more in the control than in the treatment group. We interpreted the self-efficacy results to be a positive outcome of the treatment: teachers may have used self-assessment training to depress the inflated self-perceptions of some teenagers. (Contains 2 figures, 3 tables and 2 notes.)