When its diameter goes close to or below the wavelength of the guided light, an optical microfiber/nanofiber (MNF) exhibits favorable properties such as tight optical confinement, strong near-field interaction, and excellent mechanical strength, which offers plenty of choices for combining a variety of functionalized materials ranging from semiconductors and metals to laser dyes; opens up plenty of opportunities for developing microphotonic or nanophotonic devices; and inspires new opportunities for near-field optics, nonlinear optics, and quantum optics.