Based on PISA 2009 data, this article studies the relationship between students' computer use and their achievement in reading, mathematics and science in 23 countries. After having categorised computer use into a set of different activities according to the skills they involve, we correlate students' PISA test-scores with an index capturing the intensity of use for each of these activities and with the total number of activities they perform. Overall, we find that students' PISA test scores in reading, mathematics and science increase with the intensity of computer use for Gaming activities while they decrease with the intensity of computer use for activities that are more related with school curricula (i.e. Communication and Collaboration activities; Technical Operations/Info Retrieval activities; Creation of Content and Knowledge Problem Solving activities). However, the number of activities (and hence the diversification of activities), irrespective of the intensity of computer use, is positively correlated with students' proficiency in all three PISA domains in the vast majority of countries, indicating that computers breadth of use, as opposed to intensity of use in a given activity, has some positive effect on students' PISA test scores. (Contains 3 tables, 1 figure, and 11 notes.)