Rare diseases are a major cause of morbidity and mortality in high income countries and have major repercussions on individuals and health care systems. This chapter examines the health economy of rare diseases from two different perspectives: firstly, the study of the economic impact of rare diseases (Cost of Illness studies); and, secondly, cost-effectiveness evaluation, which evaluates both the costs and results of the health care technologies applied in rare diseases. From the point of view of economics, health resource allocation is based on the principle of scarcity, as there are not - and never will be- sufficient resources for all worthy objectives. Hence, policy makers should balance costs and health outcomes. Rare diseases may well represent a significant societal burden that should rightly receive appropriate prioritisation of health care resources. As new and seemingly expensive health care technologies are developed for rare diseases, it will become increasingly important to evaluate potential and real impact of these new technologies in both dimensions: social costs and health outcomes.