The assessment of the flap perfusion in reconstruction surgery is mostly based on clinical evaluation. Unfortunately, surgeons still face some risk of complications if the flap tissue is not well perfused. Indocyanine Green (ICG) angiography allows the surgeon to define the boundaries of the perforator perfusion zones of a flap with high sensitivity, avoiding partial flap loss and fat necrosis. With the combination of preoperative Computed Tomography Angiography (CTA) and intraoperative ICG, we can define the post-dissection vascular re-balance, speed up the flap dissection and avoid post-operative flap complications. This lecture will provide you a clear understanding of the procedure and benefits of the fluorescence diagnostic method used in flap reconstruction surgery.