This chapter presents a state of the art summary of acute leukemia of ambiguous lineage (ALAL) to its definition, biology, epidemiology, disease mechanisms, clinical signs, diagnosis, treatment and prognosis. Accounting for 4% of acute leukemias, ALAL is a rare complex identity with no clear evidence of differentiation along a single lineage. Clinical symptoms associated with ALAL range from asthenia, pallor, fever, dizziness, agitation/irritability, incoordination, shortness of breath, easy bruising, excessive/prolonged menstrual bleeding, bleeding/enlarged gums, coagulation disorders, neurological disorders, severe thrombocytopenia, weakness, fatigue, and palpitations. Treatment options for ALAL range from induction chemotherapy, consolidation therapy , continuation therapy , to delayed intensification. ALAL is an aggressive disease with heterogeneous clinical, immunophenotypic, cytogenetic, and molecular genetics features. Flow cytometric immunophenotyping of cytoplasmic myeloperoxidase, CD19, cytoplasmic CD3, and other markers plays a vital role in the diagnosis of ALAL. Differential diagnoses for ALAL include myeloid antigen-positive acute lymphoid leukemia, lymphoid-positive acute myeloid leukemia, and minimally differentiated acute myeloid leukemia.