This chapter explores the Optical microfibers and nanofibers (MNFs) in terms of the fabrication techniques and optical and mechanical properties and provides an overview of some MNF-based sensors. Optical MNFs exhibit many desirable characteristics such as large evanescent field, strong optical confinement, bend insensitivity, high configurability, high compactness, and the feasibility of extremely high-Q resonators. The resulting sensors hold numerous advantages over their standard optical fiber counterparts, including high sensitivity, high detection bandwidth, fast response, high selectiveness, low intrusiveness, small size, and lightweight. Resonator-type MNF-based sensors comprise all sensors that exploit resonant structures. MNFs have been used to manufacture homogeneous resonant sensors in the arrangements of loop, knot, and coil. The sensitivity of non-resonator-type MNF-based sensors scales with the MNF length. The detection bandwidth associated with certain sensing mechanisms such as the Faraday effect decreases with longer optical path lengths. A trade-off must be considered for a good balance between sensitivity and detection bandwidth.