The wave power extraction performance of wave energy devices depends on the resonance of the water column or the oscillating buoy. For single-buoy or single-chamber oscillating water column (OWC) devices, there is typically a single efficiency peak in open water. For example, for single-chamber OWC devices, the wave power extraction depends on the movement of the water column under the action of water waves. When the resonance of the water column is triggered, there may exist a peak in efficiency. This may lead to a relatively narrow effective frequency bandwidth for efficient wave power extraction. In this chapter, we present the way to broad effective frequency bandwidth of wave energy devices by using multi-resonant technique.