This research evaluates the impact of teacher performance on the learning achievements of Peruvian students. For that purpose, we constructed a measure of teacher knowledge that includes components of subject matter knowledge and pedagogical knowledge. We apply a value added approach using longitudinal data from the 2013 Sample students Evaluation (EM 2013), which measures the learning achievements in mathematics of 6th grade children and also the knowledge of their math teachers, and the 2009 Census students Evaluation (ECE 2009), where the same students were evaluated in mathematics when they were in the 2nd grade of primary school. We find that teacher knowledge has a positive causal effect on the learning achievements of Peruvian children. This effect is heterogeneous since it is greater in children with high accumulated capacity and in children who have more educational inputs (resources) at home; that is, there is evidence of complementarity of inputs and thus educational gaps already existing in Peru (between the richest and the poorest) can be further expanded.