Inborn errors of metabolism (IEM) are a highly heterogeneous group of genetic conditions and represent a relevant cause of morbidity and mortality in the pediatric population. IEM, which are individually rare but collectively numerous, are well-recognized entities of the generic class of "rare" diseases. Since the first descriptions by Garrod at the beginning of the 20th century, several hundred new disorders have been defined, as new biochemical and molecular diagnostic tools became available. The clinical pictures of single diseases are extremely diverse, ranging from acute life-threatening manifestations to chronic late-onset forms, with single or multiorgan involvement. Mental retardation and progressive neurological impairment often characterize the clinical course. One of the principles to prevent high morbidity and mortality rates is early recognition followed by prompt therapeutic intervention. Therefore, a small number of treatable IEM is subject to neonatal mass screening. More recently, an innovative technique, based on tandem mass spectrometry, has expanded the range of neonatal screening to several additional disorders. Owing to the extreme heterogeneity, as well as to the increasing number of new disorders, exhaustive and updated epidemiological data on the overall occurrence of IEM are lacking. A national retrospective study was conducted to define the epidemiological profile of IEM in Italy and to estimate the costs related to the disease burden. Other relevant issues of our investigations focused on creating protocols of treatment for neonatal IEM, and on the development of new methods for the biochemical diagnosis.