© 2018 by The University of Chicago. All rights reserved. I analyze the effect of working while studying on the performance of Spanish students in upper secondary education. Using education and labor histories, I construct the students’ grade performance as passing, repeating, or dropping out. I estimate the effect of working while studying from the first year the students are enrolled through their year of graduation. I find that working while studying substantially reduces the probability of passing and increases the probability of repeating or dropping out during their first year of upper secondary education. Not accounting for selection into working while studying would lead to underestimating the detrimental effect of working. Sensitivity analysis shows that the results are robust.