Biochemical genetics is focused on the pathophysiology, diagnosis, and treatment of inborn errors of metabolism, which result in potentially toxic accumulation of substrate or secondary metabolites, and/or inhibition of reaction products. Early-onset metabolic disorders often present during infancy as acute, life-threatening illnesses; other forms of inborn errors appear later in childhood and are often characterized by progressive neurodegeneration. This unit provides an overview of modern biochemical genetic diagnostic methods used to evaluate a patient's metabolic status via measurement of reaction products or a specific enzyme activity. One such technique, tandem mass spectrometry (MS/MS), has revolutionized population-scale newborn screening for metabolic disorders by providing rapid, accurate, multianalyte quantification on small samples. The combination of expanded newborn screening and novel therapeutic approaches, such as enzyme-replacement therapy, offers significant improvement in the management of metabolic disorders.