The book provides the overview of the methods and tools for measuring temperature, strain, pressure, vibration and optical properties of transparent substances and materials performed by light and sensors formed in the core of optical fibers. The basic concepts of light are described, the light characteristics such as wavelength, frequency, polarization, intensity are defined. The light propagation principles in optical fibers are presented, the data transmission over optical fiber is considered, and approaches to distance measuring are given. The definition and principle of light distribution in periodic inhomogeneous optical structures are given, in particular, in the fiber Bragg gratings and Fabry-Perot resonators. Approaches to the temperature, strain, pressure, and vibration measuring are considered, as well as approaches to the properties of liquids and gases determining by optical measurements are described.