Key Inclusion Criteria

          1. Confirmed diagnosis of CLL or SLL that meets the 2008 IWCLL criteria

          2. CLL/SLL requiring treatment per 2008 IWCLL criteria

          3. Relapsed or refractory to at least 1 prior systemic therapy for CLL/SLL

          4. Measurable disease by CT/magnetic resonance imaging (MRI)

          5. ECOG performance status of 0, 1, or 2

          6. Life expectancy ≥ 6 months

          7. Adequate bone marrow function

          8. Adequate renal and hepatic function

        Key Exclusion Criteria

          1. Known prolymphocytic leukemia or history of, or currently suspected, Richter's
             transformation

          2. Clinically significant cardiovascular disease.

          3. Prior malignancy within the past 3 years, except for curatively treated basal or
             squamous cell skin cancer, non-muscle-invasive bladder cancer, carcinoma in situ of
             the cervix or breast

          4. History of severe bleeding disorder or history of spontaneous bleeding requiring blood
             transfusion or other medical intervention

          5. History of stroke or intracranial hemorrhage within 180 days before first dose of
             study drug

          6. Severe or debilitating pulmonary disease

          7. Active fungal, bacterial, and/or viral infection requiring systemic therapy

          8. Known central nervous system involvement by leukemia or lymphoma

          9. Known infection with HIV or active viral hepatitis B or C infection

         10. Moderate or severe hepatic impairment, ie, Child-Pugh class B or C

         11. Major surgery within 4 weeks of the first dose of study drug

         12. Prior treatment with a BTK inhibitor

         13. Toxicity from prior anticancer therapy that has not recovered to ≤ Grade 1

         14. Pregnant or lactating women

         15. Vaccination with a live vaccine within 35 days prior to the first dose of study drug

         16. Hypersensitivity to zanubrutinib, ibrutinib, or any of the other ingredients in either
             drug

         17. Concurrent participation in another therapeutic clinical trial

        NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.