The measurement of coupled fields in high-power microwaves is a widely used power measurement method. It involves extracting a very small portion of the extremely high power microwave through a certain coupling method for measurement, and the original power of the high-power microwave can then be calculated based on the coupling coefficient. The coupling methods for coupled field measurements can take various forms, such as probe coupling, mode-selective coupler coupling, etc. This type of measurement can be performed online and dynamically. This chapter introduces the calculation and calibration methods of the coupling coefficient for probe coupling, as well as the theory and calibration methods of the coupling coefficient for small hole coupling during coupled measurements. Additionally, we introduce various simple forms of single-mode generators here.