This Practice Note introduces and explores the complexity of the regulation of advanced therapy medicinal products (ATMPs). The Practice Note examines the concept of an ATMP, considers the regulation of ATMPs and the application of the regulatory requirements during the development of an ATMP. The note also considers borderline issues, combined ATMPs and exceptions to marketing authorisation requirements for ATMPs. The note applies the ATMP rules to the collection, storage, import and export of cells or tissues, the testing of embryonic stems cells and the manufacture, distribution, import and export of ATMPs. It also assesses the proposed changes in the ongoing reform of the EU’s pharmaceutical legislation concerning ATMPs.