Thoracic endovascular aortic repair (TEVAR) today represents the first option for the treatment of most pathologies involving the descending thoracic aorta. Proximal endograft failure, which includes endograft migration or type IA endoleak, represents the most frequent complication during the mid-term and long-term period. Proximal sealing length is the single most important factor affecting the technical success and durability of TEVAR. Other factors related to aortic arch anatomy, fluid dynamics, type of endograft, or type of pathology, may influence the risk of proximal endograft failure, and should be considered during the endovascular planning of the proximal sealing length. This review summarizes the evidence on the factors affecting the risk of proximal endograft failure, and provides the rationale for the choice of the proximal sealing length during TEVAR, based on specific patients’ characteristics.