Rare diseases affect a small number of people and are defined differently in different parts of the world as diseases that are rare in one area can be very common in another. General knowledge about rare diseases is presented in this paper, although there are still many unknowns related to this topic due to the fact that these diseases occur in a small number of people and the symptoms and signs of the disease are difficult to recognize making it difficult to define and diagnose them. An important aspect of this topic are the so-called "orphan drugs" - drugs used for therapeutic purposes in the treatment of rare diseases which are often unapproachable to patients. This paper emphasizes the health and social aspects of rare diseases with a more detailed review of the status of persons suffering from rare diseases in the Republic of Croatia, highlighting the systems in which persons suffering from rare diseases exercise their rights and services. An important role in rare diseases is played by various associations at the global and national level which contribute to increasing knowledge about rare diseases and the quality of life of patients. Also, three rare diseases are described in more detail in this paper, namely Majewski osteoplastic primordial dwarfism type Ⅱ, Hunter syndrome and Omenn syndrome.