Malignant tumors of infants and children, unlike their adult counterparts, are commonly undifferentiated or show minimal evidence of differentiation. Because of this, many of these tumors pose a formidable dilemma to the general pathologist in distinguishing one from another. A broad sampling of the more common malignant neoplasms and of newly recognized histologic subtypes which are restricted more or less to the pediatric age group is presented. The minimal microscopic criteria required to establish a diagnosis are given using widely accepted pathologic classifications and seminal journal references as a guide. Ancillary microscopic features, helpful histochemical stains, immunohistochemistry, and ultrastructural morphology are described for most tumors. Histologic distinction from diagnostic mimickers is discussed. Where appropriate, histologic grading and tumor subclassification are presented and their prognostic relevance is reviewed.