Regenerated fibre Bragg gratings are formed when specially pre-treated seed gratings are heated up to several hundred degrees centigrade. During this process, the fibre Bragg grating (FBG) reflectivity vanishes and regrows, forming a regenerated grating, which can survive temperatures of more than 1000 ◦C. Right from the beginning, it was clear that the extraordinary temperature stability of regenerated FBGs will open new fields of applications for FBGs. Multiple investigations have been made to explore the mechanisms of the regeneration effect, often presenting contradictory explanations of the processes leading to their formation. To date, there is no comprehensive theory which can explain all the observed phenomena regarding the regeneration effect. However, a broad base of knowledge exists, which can be used for tailored manufacturing and application of regenerated FBG. In this article, we present a summary of the breadth of regenerated FBG research, including a critical overview and discussion of the various competing theories published to date. We conclude with an outlook on present and future applications of regenerated FBGs and identify further research necessary to unambiguously identify and explain the key processes occurring during regeneration.