Rare diseases (RDDs) are, by definition, diseases that affect a small number of people compared to the general population. In Europe, a disease is considered rare when it affects 1 in 2,000 people. While almost all genetic diseases are RDDs, not all RDDs are genetic diseases. Today we already have some scientific and technological advances that will eventually lead to a better understanding and characterisation of RRDs and a more accurate diagnosis, and consequently a more specific and effective treatment for each patient. The diagnosis of RRDs is in many cases genetic, so that a better definition of the disease requires the application of genetic study techniques. These pages offer a review of the main genetic tests that can currently be performed in public and private centres, and discuss their contribution to a better understanding of RRDs.