The measurement of high-power microwave modes can also be achieved through another category of methods, which involves separating the various modes of the high-power microwave transmission system in the radiation space, or coupling the fields of different modes into different sub-transmission systems. Subsequently, by conducting measurements either spatially or within the different subtransmission systems, the presence and relative magnitudes of each mode can be directly determined. This approach is referred to as the mode spectral measurement of high-power microwaves. It primarily includes methods such as the dispersive reflector antenna technique, mode-selective coupling method, and wavenumber spectrum measurement method. This chapter will provide a detailed introduction to the measurement principles of these methods, the design and calibration of the measurement setups, and the techniques for mode measurement.