The burden of rare tumors in Europe is still unknown and no generally accepted definition of them exist. The Surveillance of Rare Cancers in Europe project (funded by the European Commission) aimed at providing a definition of "rare cancer", a list of cancers and rare cancer burden indicators, based on population-based cancer registry data, across Europe. An international consensus group agreed that incidence is the most appropriate indicator for measuring rare cancers frequency and set the threshold for rarity at 6/100,000/year. The list of rare cancers was based on the International Classification of Diseases for Oncology (ICD-O 3rd edition) and it was hierarchically structured in 2 layers based on various combinations of ICD-O morphology and topography codes: layer (1) families of tumors (relevant for the health care organisation) and layer (2) tumors clinically meaningful (relevant for clinical decision making and research). The burden indicators were estimated and are provided in this chapter.