Overview: Microfibers tapered from conventional optical fibers with diameters ranging from hundreds of nanometers to several micrometers possess various advantages including large evanescent field, strong light confinement, high optical nonlinearity, flexible configurability, and low-loss connection to other fiberized system, which makes it an open platform for miniaturization and integration of all-fiber devices. Nowadays microfiber can be easily obtained through mature fabrication method like flame-brushing technique. On the other hand, as a fundamental opto-electronic component, optical resonators have got comprehensively researched and widely applied in the fields of optical communication, sensing, signal processing, and quantum photonics, including whispering-gallery-mode cavities like micro-ring, micro-cylinder, micro-toroid, and micro-sphere. These traditional optical resonators are fabricated through lithography which is relatively complicated. With the maturation of microfiber fabrication methods, optical resonators based on optical microfibers have been demonstrated and developed, such as microfiber loop resonators, microfiber knot resonators, and microfiber coil resonator. As an optical coupling device based on evanescent field coupling, the microfiber resonator features in low insertion loss, high Q-factor, high finesse, excellent mechanical stability, easy fabrication process, and compatibility with fiber systems, providing a broad platform for all-fiberized miniatured devices of probing and modulation. Through further integration with exterior functional materials and microfabrication techniques, a microfiber resonator can be utilized in diverse domains of sensor, filter, modulator, and fiber laser, as well as quantum photonics and nonlinear optics, realizing the ‘lab on fiber-ring’. In the field of sensing, the microfiber resonators get exploited as the refractometric sensor, concentration and humidity sensor, temperature and current sensor, mechanical pressure sensor, microfluidic sensor, magnetic field sensor, acceleration sensor, etc., where the devices exhibit high adaptability and excellent sensitivity. As to optical signal processing, the device can be used as the single wavelength or multi-wavelength filter, code-type conversion, and optical modulation. The intensity and phase of light can be tuned to a large scale within broad wavebands, and the modulation response time is also reduced to achieve high-speed modulation. Furthermore, the microfiber resonator can be used as an optical delay line or generator of second harmonic or third harmonic. When applied into fiber laser, the microfiber resonators help build the stable light source with narrow linewidth single frequency or multiwavelength laser with high uniformity. The devices integrated with metal or 2D materials also make the laser operate under conventional soliton mode-locking or dissipative four-wave-mixing mode-locking regime and output sub-picosecond pulsation, broadening the dynamics of ultrafast optics. In this article, we summarize the recent progress in the microfiber resonators research fields, covering fundamental principles and characteristics, fabrication methods, and applications of microfiber resonators.Microfibers tapered from conventional optical fibers with diameters ranging from hundreds of nanometers to several micrometers possess various advantages including large evanescent field, strong light confinement, high optical nonlinearity, flexible configurability, and low-loss connection to other fiberized systems, which makes it an open platform for miniaturization and integration of all-fiber devices. As a fundamental opto-electronic component, optical resonators have got comprehensively researched and widely applied in the fields of optical communication, sensing, signal processing, and quantum photonics. Traditional optical resonators are fabricated through lithography which is relatively complicated. With the maturation of microfiber fabrication methods, optical resonator based on optical microfibers was demonstrated and developed. As an optical coupling device based on evanescent field coupling, the microfiber resonator features in low insertion loss, high finesse, easy fabrication, and compatibility with fiber systems. It can be utilized in domains of filter, sensor, modulator, and fiber laser. In this article, we summarize the recent progress in the microfiber resonators research fields, covering fundamental characteristics, fabrication methods, and applications of microfiber resonators.