In the frame of this thesis, we have developed a novel batch-type technology for the realization of three-dimensional millimeter-size transformers for ultra-small low-power (0.1-1 Watt) applications. The technology is based on the three-dimensional micropatterning of 0.5, 0.75 and 1 mm thick ferrite wafers by powder blasting to form the magnetic cores of the inductive devices, and the combination of these cores with electrical windings realized in flex-foil printed circuit board technology. Microfabrication and assembly of the parts can be done in a batch-process on a wafer/foil level, opening the way to further size reduction of the components. The coils are realized using an in-house developed high-resolution polyimide spinning and Cu electroplating process. Winding widths down to 5 μm have been obtained and total device volumes are ranging between 1.5 and 10 mm3. We have measured the inductive and resistive properties of our devices as a function of frequency and device geometry. The results clearly show the high potential of our technology for power applications, in which miniaturization and small-size is an issue. In the domain of RF frequencies, efforts are being made to improve the quality factor by multilayer techniques and leaving a space between the substrate and the conducting lines. Magnetic components operating at high frequencies rapidly increase eddy current and hysteresis losses in the magnetic core as the operating frequency increases. The micromachining techniques provide several approaches for the miniaturization of inductors operating at high frequencies. In the present work, we have realized millimeter-size RF inductors on silicon using the same coil as before and Ni-Zn ferrite plates. The coils are assembled with magnetic cover plates of commercially available bulk Ni-Zn ferrites of high resistivity. Using the magnetic flux-amplifying ferrite plates, we obtain a 40 % enhancement of the inductance and a 25 % enhancement of the quality factor (Q=10-20) for frequencies up to 0.2 GHz.