Acute lymphoblastic leukemia (ALL) and lymphoblastic lymphoma (LBL) constitute a biologic continuum of neoplastic lymphoid disorders characterized by the proliferation of immature (blast) cells of B-cell or T-cell lineage. Cases with tissue involvement and less than 25% replacement of the marrow cellularity by lymphoid blasts have been designated as lymphoblastic lymphomas. Cases with 25% or greater marrow involvement have been designated acute lymphoblastic leukemias. The modern approach to the treatment of ALL involves tailoring the intensity of the chemotherapy to risk groups defined by the presenting clinical features, lineage, cytogenetics, and molecular findings, as well as by the early response to therapy. This chapter reviews the clinical and pathologic features of lymphoblastic neoplasms.