During the last decade Silicon photonics emerged as mature technology in the field of integrated optics for the fabrication of low-cost optical integrated circuits [1]. Silicon-on-insulator (SOI) represents the ideal platform for the realization of optical circuits characterized by a strong optical confinement, because of the large index contrast between silicon and silicon-oxide. Moreover, using the SOI technology allows taking advantage of the reliable and well-consolidated techniques developed to process CMOS integrated circuits during more than 50 years. We report on the research activity that has been dedicated to the design and characterization of optical filters based on micro-resonators suitable to process optical WDM signals. The project was carried out in the framework of the FP7 European project “FABULOUS” (FDMA Access By Using Low-cost Optical network Units in Silicon photonics), aiming to develop low-cost optoelectronic components for new generation passive-optical-networks.