The small molecule diseases include the inborn errors of carbohydrate, ammonia, amino acid, organic acid, and fatty acid metabolism. They are central to the cohort of biochemical genetic diseases that are often associated with catastrophic presentations and life-threatening illness during infancy and childhood. Many of these entities are now routinely detected through newborn screening in the majority, if not all, of the states in the United States. Several of these diseases have effective therapies that largely eliminate the signs and symptoms of disease. In many, however, the disease process is without an effective treatment or may be brought under control but not corrected. This review contains 1 figure, 6 tables, and 11 references. Keywords: glycogen storage diseases, galactosemia, hyperbilirubinemia, hyperchloremic metabolic acidosis, hypofibrinogenemia, and thrombocytopenia, hypophosphatemia, fructose-1,6-bisphosphatase deficiency