The flexibile nature of optical fiber enables it to offer remote-access capabilities, which could be used in many biomedical applications. This review focuses on different micro- and nano-structured fiber probes for applications in biosensing, imaging, and stimulations. The modifications to fiber could extend design freedom from waveguide optimization to functional material integration. Fiber probes with optimized waveguide structures or integrated functional materials could achieve enhanced optical mode interaction with biosamples, and hence obtain ultrasensitive biosensors with a remarkably low limit of detection. Furthermore, bioimaging with a high spatial resolution can be obtained by engineering dispersion and nonlinearity of light propagation in the fiber core or designing a metal-coated tapered fiber tip with a sub-wavelength aperture. Flat metasurfaces can be assembled on a fiber tip to achieve a large depth of focus and remove aberrations. Fiber is also a compact solution to realize the precise delivery of light for in vivo applications, such as deep brain stimulation. The optical beam size, shape, and direction could be steered by the probe parameters. Micro- and nano-technologies integrated with fiber contribute to various approaches to further improve detection limit, sensitivity, optical resolution, imaging depth, and stimulation precision.