This fourth edition of Fiber Optic Sensors is revised and updated to include the new sensing technologies emerging in broad commercial use, with a focus on scattering-based distributed sensing systems. In addition, a chapter was added to describe biophotonic sensing systems and their applications. This book covers a broad range of point sensors and distributed sensor technologies and their applications in a multiplicity of markets including energy, biomedical, smart structures, security, military, and process control. It illustrates how this portfolio of technologies has addressed many sensing problems that are difficult for conventional approaches and often require survival in extremely harsh conditions. With the addition of two authors who bring 75 years of combined experience in fiber optic sensor technology, this edition is a significant update and an excellent resource for any engineer who has an interest in advanced sensing systems.