Within the past two decades the application of molecular genetic methods in paediatric endocrinology has contributed to the identification of novel disease entities and has had a major impact on our everyday clinical decisions. Therefore, the knowledge of the basic principles, methods and nomenclatures of molecular biology became an important basis for understanding the aetiology, pathogenesis and therapeutic approaches of endocrine disorders. In this chapter we have summarised the most important basic information concerning molecular biology and genetics. We discuss how to approach a patient with a genetic endocrine disease and how to counsel patients and families. Finally, an overview is given about the most common genetic disorders relevant for paediatric endocrinology. This chapter should help to understand the molecular background of endocrine diseases and to facilitate the reading of scientific publications in the field.