Precise modeling and characterization of material permittivity are essential for high-frequency devices operating above 110 GHz. However, existing data at millimeter-wave frequencies are limited, and measurements from different laboratories and techniques on the same material often vary by much as 10%. This article reviews challenges and methodologies for permittivity measurements, using high-purity semi-insulating 4H SiC—a material essential for high-frequency and harsh-environment applications—as an example. We highlight advancements utilizing Fabry-Perot resonators and substrate-integrated waveguides to measure ordinary and extraordinary permittivities, ε⊥ and ε‖, and their respective loss tangents across 55–330 GHz. Without measurement artifacts, the measured values of ε⊥ = 9.77 ± 0.01 and ε‖ = 10.20 ± 0.05 show negligible frequency dependence, while the measured tan δ⊥ of (4.9 ± 0.1) × 10−6 f increases linearly with the frequency f. The value tan δ⊥ = 3 × 10−5 is the lowest we have measured on any solid and is lower than the previous low-loss standards like sapphire. Applications in millimeter-wave circuits, including state-of-the-art D-band power amplifiers, validate these findings, underscoring their importance for next-generation communication systems such as 6G.