This paper outlines some of the major problems that can occur during the service life of hydroprocessing reactors resulting from operating at high stress levels in a high temperature and high pressure hydrogen environment. The paper discusses the influence of flaws such as weld defects, sigma phase formation in the weld overlaid ring joint grooves and at catalyst bed supports and discusses other causes of damage such as temper embrittlement, weld overlay disbonding and fatigue. The paper also outlines how reactor damage problems may be detected and assessed to determine the course of action for repair if needed. The paper outlines service life improvements of modern reactors based upon advanced materials of construction and improvements in design, fabrication and welding technology.