This chapter covers T and B lymphoblastic neoplasms, including acute lymphoblastic leukemia (ALL) and lymphoblastic lymphoma (LBL). Morphologic, immunophenotypic, and cytogenetic features are discussed along with the parameters of various modalities used in their diagnosis. The techniques and utility of minimum residual disease (MRD) measurement after therapy is discussed, as are new insights into the molecular biology of leukemogenesis gleaned from recent studies using gene expression profiling and copy number analysis via microarray techniques.