This is a Phase 2, open-label, multicenter study, evaluating the efficacy of venetoclax in participants with relapsed or refractory Chronic Lymphocytic Leukemia (CLL)/Small Lymphocytic Lymphoma (SLL) either in presence of 17p deletion (Cohort 1) or those who have failed a B-receptor signaling pathway inhibitor (BCRI) therapy and who have also failed, or were unable to receive chemoimmunotherapy (CIT) irrespective of 17p status (Cohort 2).