The medical care of people affected by rare diseases (RD) represents a significant burden and an additional challenge for health systems, compared to some common diseases. The specific characteristics of RD, such as the low number of patients, their diagnostic complexity, high morbimortality and lack of specialised knowledge, among many others, together with the methodological difficulties derived from them, make these conditions a distinct group of scientific interest and high impact. The more traditional epidemiological research aimed at identifying and distributing the determinants of RD continues to be a pending issue, which limits scientific knowledge and, above all, the planning of resources and their cost, making this group of diseases a public health problem.