Inborn errors of metabolism, though individually very rare, have a collective incidence of 1 in 50. The common disorders usually presenting in adults are hereditary haemochromatosis (1 in 200) and familial hypercholesterolaemia (1 in 500); others occur with a frequency of 1 in 5000 to 1 in 5 million. The incidence is of course much higher in certain ethnic populations, due to consanguinity. Yet, there is an ostensible lack of effort on the part of the clinical community as a whole, to develop any degree of understanding of their presentation, diagnosis and principles of management. Many of them are treatable by relatively simple dietary manipulations (vitamins or cheap drugs e.g. Zinc acetate for Wilson’s disease); unrecognised and untreated, the morbidity and mortality is unacceptably high. Due to rapid advancements in biochemical techniques e.g. tandem mass spectrometry, preclinical diagnosis is possible by neonatal screening programmes, making it possible for babies with conditions like phenylketonuria and medium-chain acyl CoA dehydrogenase deficiency to potentially have near normal neurological development and a good quality of life. At the same time there have been significant advances in treatment options such as synthetic dietary supplements (pheynylketonuria; maple syrup urine disease), enzyme replacement (lysosomal disorders) and whole organ transplants (lung and pancreatic transplants for cystic fibrosis); gene therapy is also knocking on the doors. In this book we attempt to outline common inborn errors of metabolism, their presentation, diagnosis and management and hope to stimulate the reader to explore more detailed texts to develop a specialised interest.