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Mutual coupling between radiation frequency (RF) components, antenna elements in a microstrip antenna array or between two microstrip antenna arrays is a potential source of the performance degradation. The degradation includes the impedance mismatching, the increased side-lobe level, the deviation of the radiation pattern from the desired one, and the decrease of gain due to the excitation of the surface wave. In the application of continuous wave radar, when the transmitting antenna and the receiving antenna are placed closely side by side, the transmitting energy may even blockade the receiver. In the application of adaptive nulling arrays, the mutual coupling will result in the serious degradation of the adaptive nulling performance. The mutual coupling is also important to the issues of electromagnetic compatibility (EMC) when a lot of RF systems work simultaneously in a limited space, especially to the signal integrality (SI) design. The mechanism of the mutual coupling is quite complicated which involves both the near field and the far field couplings. In dealing with these problems, the full wave analysis is indispensable. In the meantime, in the analysis or optimization design, the frequency response should be taken into account as well. In this paper, the effective frequency dependent full wave analysis will be introduced. Examples of mutual coupling reduction are given that involve the mutual coupling reduction between two microstrip antenna arrays and the mutual coupling reduction between microstrip antenna elements,. In addition, the mutual coupling effect on the performances of some antenna arrays will be briefly addressed.