Cut- Outs throw the fuselage are very common in commercial aircrafts. One of the most critical Cut- Out that an aircraft has in its fuselage, is the one used to mount the Horizontal Stabilizer because a very high load concentration in this region occurs. Therefore, additional structures such as splices and beams must be installed to carry the loads around the openings. This Master Thesis studies the mechanical behaviour of the Cut – Out of the Horizontal Stabilizer with a Finite Element Model and provides the static sizing and fatigue check of the parts related to it. The analyses performed on the structure are: static strength of the splices, damage tolerance at skin, bolted joints analysis and fatigue check of the splices. The results show a very small reserve factors in this region. Therefore, this Cut- Out must be treated as critical, especially during the manufacturing of the aircraft but this also implies a good design because it will withstand all the known load cases and it is as light as possible.