This chapter focuses on the recent developments in the area of fiber-optic sensing from the perspective of physical scale including an overview of the fundamentals of the phenomena utilized for sensing at different physical scales along with some specific examples of macro- and micro-/nanosensors and their practical applications. It also discusses future challenges and opportunities for fiber-optic technology at different physical scales and some emerging applications of fiber-optic sensors. Optical fibers have revolutionized communications industry and the early 1970s have become the communications medium of choice for telephones, Internet, cable television, security cameras, utility, and industrial networks. The distinctive feature of the fiber-optic technology is its dual functionality, as the fiber can serve as both the sensor and the telemetry channel. Commercial interrogators for multiple fiber-optic sensors fall into two main categories: Time Division Multiplexing (TDM) and Wavelength Division Multiplexing (WDM). Biosensors for medical applications will be an important application field of optical fiber sensors.