This Green Book provides a comprehensive guide to transformer and reactor life management, from procurement to disposal. Transformers and reactors are among the most expensive components in the power system and contribute to a large proportion of its losses. Transformers also have long lives - more 40 years in many cases. Making the wrong decisions on their life management can have serious and long-lasting consequences. The book is a reference for anyone involved in transformer and reactor life management. This includes not only operators, but also maintenance, repair, testing, and disposal contractors. Each of the main steps is described in its own chapter, with special emphasis on diagnosing and resolving transformer and reactor problems. Each chapter has been written by experts in the field, and then reviewed in detail by the editorial panel. In addition, the editorial panel has tried to ensure a clear and consistent use of terminology. The book provides those involved in transformer and reactor life management with comprehensive guidance on industry best practices and how to avoid wrong decisions. Readers who would like to comment on any of the published books or identify errors to the editorial team please contact:  cigregreenbooks@springer.com .