Childhood non-Hodgkin lymphomas are relatively common childhood cancers composed of malignant B lymphocytes or thymocytes at various stages of maturation, usually intermediate or late. Most often, they have a diffuse pattern of infiltration and are high grade, lymphoblastic, small noncleaved-cell (undifferentiated, Burkitt or non-Burkitt types) or large cell types. Primary site correlates with immunophenotype and histopathology. For example, most abdominal tumors are B cell and non-lymphoblastic types, and most thoracic primary tumors are of thymocytic origin. Skin presentations are rare, usually lymphoblastic in histology and often of the pre-B cell phenotype. An appropriate work-up and adequate staging is mandatory, since the selection of therapy depends heavily on disease stage. Histopathologic findings are additionally important in determining appropriate therapy for advanced-stage disease. Finally, cytogenetic studies of B and T cell malignancies have defined nonrandom abnormalities in many of these neoplasms and, together with molecular analyses, hold exciting promise for an improved understanding of tumor pathogenesis.