Microfiber (MF) devices are increasingly becoming attractive building blocks for numerous applications. This is attributed to a combinations of enabling properties including large evanescent field, compactness, low insertion loss and, most importantly, their configurability. This thesis describes the evanescent field coupling in MF devices in different forms and applications, such as microresonators for sensing, modal couplers for mode conversion and nonlinear frequency conversion. Micro-resonators are devices based on evanescent field coupling between turns of adjacent segments of the MF. Light inside these devices is confined within the resonator structure and trapped inside its cavity. Light, therefore, follows a longer path before passing to the output and this is wavelength dependent. A new type of resonator was successfully designed and manufactured: the multiport MF coil resonator. This structure has excellent features such as high extinction ratio, multiple resonance peaks and high stability. Novel modal couplers were also analysed and fabricated with high conversion efficiencies, high modal purity and low insertion loss. Modal couplers were manufactured to convert the launched input LP01 mode, into any of the higher order modes supported by the fiber. High conversion efficiencies were demonstrated for the LP11,LP21 and the LP02 modes. A MF coupler was successfully used for the detection of DNA by exploiting hybridisation on the surface of the MF coupler. The large evanescent field of the MF allows to detect the refractive index change that occurs when DNA attaches to the surface. Detection limits in the range of 10⁻⁶ per Refractive Index Unit (RIU) and sensitivities as high as 200nm=RIU were obtained. Nonlinearities were investigated both in straight MF and MF resonators, with particular stress on the second and third harmonic generation. While the conversion effciency remained relatively low for straight MFs, by using a loop resonator, the circulation of pump power (at resonance) inside the loop allowed for a conversion efficiency enhancement of 7.7 dB for the Third harmonic generation (THG) and 7.6 dB for the Second Harmonic Generation(SHG). Knot resonators were also used and provided larger enhancement of 14.2 dB for the SHG signal.