This review considers current knowledge regarding the incidence of post-operative atrial fibrillation (POAF) after cardiac surgery and during the subacute post-operative period (e.g., ≤ 3 months post-surgery), the relationship between POAF and clinical outcomes, and the evidence regarding the use of oral anticoagulation (OAC) in patients with POAF after cardiac surgery. The interpretation of studies examining POAF should consider how each study defines POAF, the threshold duration of POAF, how POAF was detected, and when POAF was evaluated following cardiac surgery. Indeed, inconsistencies in how POAF is defined and detected may explain the wide variation reported for the incidence of POAF after cardiac surgery. The use of more sensitive rhythm monitoring will be more likely to detect POAF, but the benefits of using these time-consuming and costly techniques versus usual care after cardiac surgery are not well defined. Further research is required to provide insight into the benefit of enhanced cardiac rhythm monitoring for POAF versus usual care after cardiac surgery.