Mode measurement is a unique and highly important aspect of high-power microwave testing. The most direct method for mode measurement involves determining the microwave’s mode or mode composition by measuring its field distribution. Techniques for measuring field distribution include the scorched spot method, thermosensitive paper (film) method, neon tube array method, liquid crystal film method, analytical method of thermal image and others that directly display field distribution images. Alternatively, the field distribution of the mode can be measured in space or within a waveguide, and the mode composition can be deduced through analysis and calculations of the field distribution. This chapter provides a detailed introduction to the various measurement and analysis methods mentioned above, as well as techniques for mode identification.