Although common in the neonatal intensive care unit, eosinophilia is often overlooked or ignored. The latter might be, at least in part, because evaluating the neonate with eosinophilia can be a complex process. This article reviews the physiologic features of eosinophilia, reference ranges, and clinical conditions associated with eosinophilia in the neonate. Recommendations for the evaluation of the neonate with eosinophilia are presented.