The interrelationship between secondary school student confidence and attainment in science is a phenomenon that has been well documented by the science education community. The purpose of this action research intervention was to acquire an in depth knowledge of the relationship between self-confidence and academic attainment, and to explore the impact that a confidence based assessment for learning (AFL) could have on student self-confidence and attainment in science. The study was implemented during a five lesson sequence on Collision Theory with a representative top-set Year 10 class. A review of the literature indicated that a significant correlation exists between assessment, student self-confidence and attainment. The analysis of the results of the impact of confidence based AFL showed that the confidence of each of the students in the science classroom is significantly increased, yet altogether each student’s attainment remains similar. Further analysis using a questionnaire suggested that students with the lowest confidence levels in science could in fact experience the greatest increase in their self-confidence in assessments through the use of regular confidence based AFL.