Legislation on the marketing of seed and plant propagating material, often referred to as ‘seed legislation’, specifies the requirements that seed and other propagating material must fulfil to be marketed legally, and how this marketing may be conducted. Such legislation can have a great impact on the composition of the seed market, as well as on cultivation and breeding, not least as it has the potential to restrict access to and maintenance of crop genetic diversity. This article presents and discusses seed legislation in the European Union (EU) and its relationship to crop genetic diversity. In the EU seed legislation is based on the principles of variety registration and certification of seed lots. Seed may be marketed only if it belongs to a variety that has been registered and the seed lot has been certified. A variety must satisfy distinctness, uniformity and stability requirements. For heterogeneous varieties this can be problematic, which in turn has potential consequences for the maintenance and further development of crop genetic diversity.