This paper develops useful tools to address resonance phenomena in islanded microgrids (MGs). The modeling of islanded MGs is first proposed based on simplified passive circuits where both Thevenin's and Norton's equivalent representations are considered to describe the dynamic of the interconnected distributed generators (DGs) and loads. It is shown that the interactions between the interconnecting passive filters as well as the presence of disturbing loads leads to complex resonance phenomena at various frequencies, which reduces power quality and even leads to instability. To cope with these issues, an active damping method based on a robust disturbances observer is proposed. The proposed method is designed to mitigate resonance issues encountered in islanded MGs with multiple electronically interfaced DGs and loads. This damping method is seamlessly incorporated into the conventional hierarchical control structure through the direct control signal modification. The main merit of the proposed approach is to calculate the appropriate resonances compensating signal without prior knowledge of the system parameters and without affecting the control bandwidth. Simulation and experimental results are provided to show the effectiveness of the developed damping approach.