Intro: Chest Tube Insertion (CTI) should be trained in simulated settings prior to patient contact. Feedback and certification is based on valid assessments, especially in simulation-based training. This study aimed to develop a novel assessment tool for CTI and to ensure content validity based on expert opinion collected through a structured Delphi study. Methods: A diverse European expert panel was invited to participate. In round 1, the experts provided at least five procedural steps and three errors involved in CTI. Round 2 evaluated the level of agreement with the inclusion of each item in the assessment tool on a five-point Likert scale. In round 3, experts rated their agreement on inclusion of the procedural step with its descriptive anchors. Consensus was reached when ≥80% of participants agreed on an item's inclusion. Results: Thirty-six of 105 (34%) invited surgeons (26/75, 35%), pulmonologists (8/23, 35%) and emergency physicians (2/7, 29%) participated. The overall response rate was 81% (29/36): 100% (36/36) in round 1, 83% (30/36) in round 2, and 97% (29/30) in round 3. Round 1 resulted in 23 steps and 44 errors after condensation and removal of duplicates. In round 2 consensus was achieved for 15 steps (65%) and 14 errors (32%). Nineteen steps were adapted into a rating scale with descriptive anchors and a list of 16 errors was presented to the panel. In round 3, experts reached consensus on the inclusion of 17 procedural steps (89%) with descriptive anchors and on all 16 errors. Conclusion: A multidisciplinary expert panel achieved consensus in the development of the ACTION (Assessment of Competence in Chest Tube Insertion) tool. This procedure-specific rating scale of 17 steps, supplemented with a checklist of 16 errors, requires further research to collect validity evidence.