This is a multi-center, single-arm, open-label, Phase 2 study of duvelisib, an orally bioavailable dual inhibitor of PI3K-δ,γ, in patients with CLL/SLL who have previously been treated with ibrutinib or another Bruton's Tyrosine Kinase Inhibitor (BTKi) and relapsed or were refractory to such therapy or discontinued such therapy due to toxicity.