Although during the last decade the number of computers at secondary schools has increased considerably in many countries, only a small percentage of teachers are using computers for instructional purposes. In the COMPED study principals, computer coordinators and teachers were asked to indicate the obstacles which are hampering the introduction of computers in their school and classroom practice. The results which are based on data from France, Japan, the Netherlands and USA, show that the most important problems are: lack of hardware, software, knowledge and time. These problems in implementing computers in education are at the same time the most important reasons why non-users do not use computers for instructional purposes. A comparison between the relative most intensive computer-using schools and the less intensive users shows that both groups have mostly the same problems as mentioned before. Besides, schools with a high level of computer use have more often than other schools overcome organizational problems.