Microfiber is an optical fiber with a diameter of several micrometers or even a hundred nanometers, and is generally obtained by heating, melting and stretching standard commercial optical fibers. Microfiber has many excellent characteristics, including strong evanescent field, high nonlinearity, high mechanical strength, low insertion loss, etc. These features, for the miniaturization of fiber optic devices and functional integration, provide a platform with high degree of freedom. By integrating various external materials, preparing a wide variety of artificial micro-nano-structures, and discovering endowed nonlinear properties, the functions and applications of microfiber have unlimited possibilities. The integration of multiple functions can be achieved on a single microfiber, making microfiber-based lab-on-fiber possible. In this chapter we present the relevant advances in this field in terms of the basic properties of microfibers, key device implementations, external integration, and applications.