Inclusion Criteria:  The subject, parent or guardian must voluntarily sign and date an informed consent. Subjects must have relapsed or refractory acute lymphoblastic leukemia (ALL) or relapsed or refractory lymphoblastic lymphoma (LL) and have exhausted available therapies of known benefit for ALL/LL. Refractory is defined as persistent disease after at least 2 courses of chemotherapy. Subjects must be ≥4 years of age Subjects must weight ≥20 kg  Subjects must have adequate hepatic function:  a. ALT and AST ≤5 x ULN and bilirubin ≤1.5 x ULN  Subjects must have normal creatinine for age or have a calculated creatinine clearance ≥ 60mL/min/1.73m2  Subjects must have adequate performance status:  Subjects ≤ 16 years of age: Lansky ≥ 50, Subjects > 16 years of age: Karnofsky ≥ 50 or ECOG <3 Female subjects of childbearing potential (those who are not postmenopausal for at least 1 year or surgically sterile by bilateral oophorectomy, salpingectomy or hysterectomy) and their male partner must practice as least 1 method of birth control during treatment and through at least 30 days after the last dose of investigational drugs. Male subjects who are sexually active with women of child bearing potential must agree to use condoms during treatment. Female subjects of childbearing potential must have negative results for serum or urine pregnancy test.  Exclusion Criteria:  Subjects who have CNS disease with cranial involvement that requires radiation Subjects who are less than 100 days post-transplant, or >100 days post-transplant with active Graft-versus-host disease (GVHD), or are still continuing post-transplant immunosuppressant therapy within 7 days prior to the first dose of investigational drug.  Subjects who received any of the following prior to the first dose of investigational drug:  A strong or moderate CYP3A inhibitor or inducer within 7 days Aspirin within 7 days Subjects who have active, uncontrolled infection Subjects who have not recovered to less than Common Terminology Criteria for Adverse Events (CTCAE) grade 2 from clinically significant adverse effect(s)/toxicity(s) of previous therapy. Subjects with malabsorption syndrome or any other condition that precludes enteral administration. Female subjects who are pregnant or breastfeeding. Male subjects who are considering fathering a child within approximately 30 days or donating sperm during treatment, within approximately 90 days after the last dose of venetoclax/navitoclax.