Lymphoblastic leukemia/lymphoma (ALL/LBL) is a genetically heterogenous group of hematologic malignancies affecting both pediatric1,2 and adult patients.3, 4, 5, 6 A majority of ALL/LBL (approximately 80 %) are of B-cell phenotype, presenting primarily as leukemia (B-ALL, in bone marrow usually with peripheral blood involvement) with less common presentations in lymph nodes and extranodal sites (B-LBL).5,7,8 The remainder (approximately 20 %) of ALL/LBL are of T-cell phenotype and also commonly present as leukemia but with a higher proportion of presentations as lymphoma (T-LBL) in the thymus gland, the site of T-cell maturation, as rapidly enlarging anterior mediastinal masses, an oncologic emergency.1,9 Rare ALL/LBL cases show an NK cell-like phenotype. This review article discusses 1) recent key developments in the diagnostic subclassification of B-ALL and T-ALL, organized by genetic driver and dictating prognosis and therapy, and 2) current laboratory approaches to minimal/measurable residual disease (MRD) assessment for these two sets of diseases. The intended audience includes general hematopathologists, general surgical pathologists practicing some hematopathology, and trainees and technologists interested in hematopathology.