This document summarizes acute lymphoblastic leukemia (ALL) in children. It discusses the classification of ALL which includes morphological, immunological, and genetic classification. Morphological classification includes the FAB system which defines three categories of lymphoblasts. Immunological classification is based on monoclonal antibodies and distinguishes between B-lineage, T-lineage, and myeloid phenotypes. Genetic classification identifies cytogenetic abnormalities which are important for diagnosis, risk assessment, treatment and prognosis, with some abnormalities associated with favorable prognosis and others with poor prognosis.