Organic acidurias are inherited metabolic diseases due to the deficiency of an enzyme or a transport protein involved in one of the several cellular metabolic pathways devoted to the catabolism of amino acids, carbohydrates or lipids. These deficiencies result in abnormal accumulation of organic acids in the body and their abnormal excretion in urine. More than 65 organic acidurias have been described; the incidence varies, individually, from 1 out of 10,000 to >1 out of 1000,000 live births. Collectively, their incidence approximates 1 out of 3000 live births. Among these disorders, methyl malonic aciduria, propionic aciduria, maple syrup urine disease and isovaleric aciduria are sometimes referred to as classical organic acidurias. In this review, we focused on the basic GC-MS-based methodologies employed in the diagnosis of classical organic acidurias and provided updated reference values for the most common involved organic acids. We also attempted to provide the most recent updates on the pathogenetic bases of these diseases.