It is estimated that around one third of admissions to paediatric wards are due to conditions with some genetic component and therefore an understanding of basic genetic principles is becoming increasingly important. In addition to describing these basic genetic principles, this chapter describes patterns of inheritance, the new developments in molecular genetics which are used for diagnostic purposes and the clinical features of the more common genetic disorders which may be encountered in paediatric practice. The chapter includes many diagrams and clinical photographs to illustrate concepts which can sometimes be difficult to follow from text alone and to introduce the reader to the field of dysmorphology and patterns of malformations.