Homework has traditionally been considered positive for students' academic achievement, to the extent that it makes children more responsible while learning. Nevertheless, making students do a large amount of homework has been one of the most criticised practices in recent years. Parental associations have long held the view that Spanish elementary school pupils spend too much time on homework. This situation is so alarming that it has recently caused a 'homework strike', which has been supported by many children's parents. Furthermore, being assigned an excessive amount of homework is a serious problem which extends well beyond Spain. In light of this context, this article aims to provide new evidence on the relationship between homework and academic performance. To carry out this research, rich longitudinal census data, together with a twin fixed-effects approach and value-added models, have been used. In this way, we can control for variables that are the same within twins, allowing us to generate a better understanding of the association between homework time and primary children's academic achievement. Our main finding is that homework is not associated with young people's academic achievement (at least within this specific country context).