This paper is based on experiences with remedial online learning from a national collaboration initiative in the Netherlands involving the University of Amsterdam, Erasmus Rotterdam University and Maastricht University (www.web-spijkeren.nl). The central question is how prior knowledge tests and online remedial summer courses can contribute to mitigating the problems of heterogeneous student enrolment. Although the insights gathered for the paper are from pilots for first-year Bachelor's programmes, the guidance they provide on how to implement an online summer course programme successfully should prove valuable to other organizations. (Contains 1 figure, 2 tables and 6 endnotes.)