In the 13 years since it was amended to require rational safety assessments of aircraft systems, Federal Air Regulation Part 25.1309 has had a significant impact, both positive and negative, on system design and maintenance practices in the civil aircraft industry. The regulation has fostered genuine system engineering and has provided the only viable methodology available for assessing and certifying some new critical and complex aircraft functions. At the same time it has perturbed long-standing, historically proven design and maintenance concepts. It has generated much controversy, notably over the use of quantitative probability for assessing risk, and has been widely misunderstood both by industry and the FAA. These and other positive and negative effects of the regulation are discussed in this paper, together with the background leading to the philosophy and methods of compliance presently embodied in the regulation and its related guidance material.