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Palladium-based fiber-optic sensors have been one of the most promising configurations for hydrogen sensing. In the latest decade, fiber-optic sensors have indeed earned a strong interest owing to their ability to monitor molecular hydrogen at specific spatial points-either as a sensing tip device or in large areas via multiple sensing regions distributed along the optical fiber. This review focuses on the various types of optical fiber hydrogen sensors, containing specifically palladium as active element. Three distinct working principles are described, viz. interferometric-, in tensity-, and fiber grating-based sensors; their characteristics and sensing performances are critically overviewed.