Introduction: The research has two objectives--to characterise college students who combine work and studies, and their jobs, on one hand; and to analyse the effect of work on various measures of academic performance, on the other. Method: The study is developed using individual information derived from a survey (N = 464) and from university records. We have estimated models that consider diverse measures of academic performance and control for a wide set of factors (including socio-economic characteristics and diverse measures of motivation and academic effort), distinguishing between different levels of intensity in the labour activity. The techniques used are logistic regression, ordinary least squares and instrumental variables. Results: The evidence obtained confirms, first, that a noticeable proportion of Spanish university students have a paid job, and often work regularly for extended periods. It also is common for those who work to do so motivated by necessity, and to perform tasks unrelated to the content of their studies. Moreover, the likelihood of working is higher among older students, foreigners and those who have higher levels of motivation. Secondly, the results suggest that, in contrast to the perceptions expressed by students, working regularly does not appear to have a significant impact on their academic performance. Conclusion: The article provides additional evidence on a very relevant issue that has however received very little attention in the Spanish case and for which neither theoretical studies nor previous empirical research have reached conclusive results. The article provides useful support for the consideration of employment-related issues in the designing of the itineraries followed by university students.