Abnormal serum α-fetoprotein (AFP) levels are frequently observed in common disorders such as spina bifida or Down's syndrome in the fetus and cancer in children and adults. The focus of this chapter summarizes on the role of serum α-fetoprotein (AFP) as a useful biomarker in malignant pediatric tumors. The fetal yolk sac and liver generate high levels of AFP during gestation and decline over the next 12 months of infancy, and only trace amounts are detected in childhood. As a result, persistent elevation of AFP correlates with a number of select pediatric malignant conditions. Serum AFP is overexpressed in a considerable fraction of germ cell tumors (GCTs), hepatoblastoma (HB), and hepatocellular carcinoma (HCC). We provide the reader with a review of AFP as a useful specific marker for the diagnosis, management, and followup in select pediatric cancers.