With the rapid progress of high-speed communication as well as millimeter-wave and sub-terahertz (sub-THz) technologies, polymeric insulating materials with low dielectric constant (D_k), low dielectric dissipation factor (D_f), and tunable molecular structures are increasingly required in the tens-to-hundreds of GHz range. In recent years, researches on the high-frequency dielectric properties of aromatic and fluorinated polyimides (PIs) have expanded from simple parameter reporting to multiple perspectives, including polarization mechanism analysis, molecular structure control, humidity dependence, and correlations with optical properties. This review provided a systematic overview of the dielectric behavior of PIs above 10 GHz, with a focus on the dominant frequency regimes of dipolar and electronic polarizations, the influence of humidity on D_k and D_f, and the modulating effects of fluorine content, free volume, and polar functional group fraction on dielectric dispersion. While ensuring the thermal stability, chemical resistance, and mechanical reliability, the review further summarized molecular design strategies for reducing D_k and D_f, and proposed future directions for material optimization toward millimeter-wave and sub-THz applications.