Chronic lymphocytic leukemia (CLL) is the most common leukemia in Western countries. The purpose of this study is to assess how well ABBV-453 works adult participants with relapsed/refractory (R/R) untreated CLL/small lymphocytic lymphoma (SLL). Adverse events, pharmacokinetics, and change in disease activity will be assessed. ABBV-453 is an investigational drug for the treatment of CLL and SLL. There are 2 parts to this study. In part A participants will be placed 1 of 5 cohorts with a specific target dose for each cohort and receive obinutuzumab during the debulking period followed escalating doses of ABBV-453, until the appropriate target dose is achieved. In part B participants will be placed in 2 cohorts and receive up to the maximum dose in part A, with cohort 2.1 including a debulking period (obinutuzumab) as in part A. Approximately 80 adult participants with previously R/R CLL/SLL will be enrolled in the study in approximately 40 sites across the world. Participants in part A will placed into 1 of 5 cohorts with a specific target dose for each cohort and will receive intravenous (IV) obinutuzumab as part of the debulking period, followed by escalating doses of oral ABBV-453 until the appropriate target dose is achieved. Participants in part B will be place in one of 2 cohorts. Participants in cohort 2.1 will receive IV obinutuzumab as part of the debulking period, followed by escalating doses of oral ABBV-453 until the maximum target dose from part A is achieved. Participants in cohort 2.2 will receive no treatment during the the debulking period, followed by escalating doses of oral ABBV-453 until the maximum target dose from part A is achieved. The estimated study duration is 5 years. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, and checking for side effects.