A probing report on the harm caused by hard-core cartels, on progress made in combating them, and on the challenges that lie ahead. Since adoption of its 1998 recommendation on effective action against hard-core cartels, the OECD has been leading an international effort to restrict their activities. This report reviews progress in combating hard-core cartels notably through strengthened sanctions against companies and individuals. It also quantifies the harm cartels cause and identifies improved methods of investigation. Despite recent progress in several jurisdictions, much remains to be done and this report outlines and identifies the challenges that lie ahead.