Assessing damages in follow-on actions against cartels that have infringed EU or domestic antitrust law is still in its infancy in Europe. This Article highlights the reasons why this issue has only recently started reaching the courts and analyses the pitfalls and problems courts face when calculating or estimating the amount of loss caused by a cartel to individual plaintiffs. As this endeavour raises complex economic questions, the Article outlines the basic economic approaches to measure antitrust harm and derives general principles from EU law on how courts should deal with economic evidence in follow-on actions to ensure an effective enforcement of competition law.