This chapter presents the current status of rare diseases in the field of public health. It briefly presents the different definitions of rare diseases worldwide, highlighting the pivotal role of the definition provided by the Orphan Drug Act in the USA, and provides a picture of these diseases as a whole. It discusses three hypotheses regarding the emergence of this category of diseases. They could have appeared as a result of the development of biomedicine, as a result of the advancement of genetics or as a result of patients mobilization. This chapter ends with a description of the sample of diseases the book draws on for empirical analysis, and for which patients were interviewed and organizations studied, in France: cystic fibrosis, fragile X syndrome, mastocytosis, Wilson’s disease, locked-in syndrome and a very rare syndrome.