The purpose of this open-label, single-arm study is to evaluate the efficacy of venetoclax monotherapy in approximately 250 participants with relapsed/refractory CLL including those with the 17p deletion or TP53 mutation (assessed by local lab) OR those who have received prior treatment with a B-cell receptor inhibitor. The starting dose of venetoclax is 20 mg once daily. The dose must be gradually increased over a period of 5 weeks up to the daily dose of 400 mg. Participants may continue receiving venetoclax for up to 2 years. After the treatment period, participants may continue on into a 2-year follow-up period.