The European population is strongly affected by cancer. Radiotherapy is roughly used in 50% of cancer patients in European countries. The increased cancer burden demands a new generation of radiation/clinical oncologist (RO/CO) that, besides a strong evidence-based oncological knowledge, will be ready for leadership in cancer care. The mutual recognition of professional qualifications of Radiation/Clinical Oncology in the EU needs training harmonization. The European Society of Radiotherapy and Oncology (ESTRO) and the European Union for Medical Specialties (UEMS) made important efforts toward a European Common Curriculum for RO/CO leadership in cancer care. If qualifications are mutually recognized, the training supporting these qualifications should be also harmonized. Since 1991, ESTRO produced several editions of the Core Curriculum in Radiation Oncology (1991, 2004, 2012, 2019). These Core Curricula were endorsed as European Training Requirements by the UEMS in 2004, 2013, and 2019. A core curriculum for clinical oncology was also produced to provide this harmonization tool to countries where radiation oncology is practiced inside the broader specialty of clinical oncology. New initiatives are in place to continuously adapt the training programs to the rapidly evolving cancer care organization.