Post Operative Atrial Fibrillation (POAF) is the most common complication of cardiac surgery. POAF incidence can exceed 50% depending on the patient baseline characteristics and surgery type. Patients with POAF tend to have worse acute and long-term clinical outcomes. BoxX-NoAF is a randomized trial to evaluate if prophylactic ablation and exclusion of the Left Atrial Appendage at the time of other routine cardiac surgery can reduce the incidence of post operative AF and clinical AF during long term follow up in patients who have not yet developed AF but are at risk.