This is a Phase 3 global, randomized, open-label study comparing LOXO-305 (Arm A) to investigator's choice of either idelalisib plus rituximab or bendamustine plus rituximab (Arm B) in CLL/SLL patients who have been treated with at least a covalent BTK inhibitor (BTKi). Patients may have discontinued the prior covalent BTKi due to disease progression (PD) or intolerance. Patients who have received venetoclax are eligible for the study. Eligible patients will be randomized in 1:1 to Arm A or Arm B.