The past 3 decades have witnessed impressive progress in our understanding of inherited neurometabolic diseases, promoted by the rapid development and application of molecular genetic strategies. Such progress has required the juxtaposition of clinical evaluations and basic science techniques. The central role of careful and complete assessment of affected children cannot be overemphasized and in no way has been diminished by technologic advances. Indeed, enhanced clinical and laboratory evaluations have led to important conceptual advances. Molecular genetics has elucidated those disorders with known metabolic defects through functional cloning and explained the variability of disease expression based on specific mutational events. Alternatively, positional cloning has identified molecular defects for those disorders with clear phenotypic patterns, but lacking a defined metabolic abnormality. Regarding heterogeneous expression, disorders with clearly different phenotypes can arise from different mutations within the same gene. The multiple variants of beta-hexosaminidase deficiency (Tay-Sachs disease) are, arguably, the best examples. Conversely, disorders with similar phenotypes are explainable by quite different mutational events. In addition, the identification of specific diseases exhibiting both biochemical abnormalities and disturbed organogenesis has blurred conventional dogma regarding separation of genetic disorders into strict metabolic and structural categories. Disorders of peroxisomal function and the neuronal ceroid lipofuscinoses are prototypes for the points noted above and raise important issues regarding our approaches to children with these disorders. These issues include a high index of suspicion for an inherited neurometabolic disease and an open mind to possible interrelations with other known and seemingly dissimilar conditions.