Wireless communication, precision guidance, imaging, internet of things, and biomedical applications in the millimeter-wave and THz bands promise new functionalities that cannot be provided by present systems. Different from present design methods, new design methods are required to support the development of high-performance millimeter-wave and THz band devices and systems. These new design methods should incorporate new materials, advanced processing and technologies, and can accurately account for multi-physics coupling effects. This special issue collects 32 papers including 20 papers on active device modeling, nine papers on passive device and circuit modeling and design techniques, and three papers on emerging system applications. Active devices for state-of-the-art millimeter-wave and THz systems discussed in this special issue include Indium phosphide (InP) heterojunction bipolar transistors (HBTs), Indium Gallium Phosphide (InGaP)/Gallium arsenide (GaAs) HBTs, Gallium nitride (GaN) high electron mobility transistors (HEMTs), GaAs HEMTs, complementary metal oxide silicon (CMOS) transistors, and electric vacuum devices. The discussion covers the important device features of high frequency, high power, low noise, and high efficiency. As the frequency goes up to the millimeter-wave and THz bands, advanced physical-based or SPICE-like equivalent circuit and compact models are needed. As a solid-state device, InP transistor can operate at a very high frequency by taking advantages of extremely high electron mobility. Zhang et al1 presented a review on the compact modeling of InP HBTs for THz integrated circuits. Useful improvements made for HBTs are reported on the analysis of intrinsic base resistances by Chen et al,2 on large-signal models by Hu et al,3 on the determination of cutoff and maximum oscillation frequencies by Zhang and Gao,4 and on the thermal resistance calculation by Wang et al.5 Due to a high breakthrough voltage and saturation velocity, GaN HEMTs is very promising for millimeter-wave solid-state power amplifiers. Chen et al6 reported an improved quasi-physics zone division large-signal model to account for electro-thermal effects, which is valid for the ambient temperature range of 245 to 390 K. Physical parameters' effects, reliable parameter extraction, and dynamic thermal impedance extraction for the equivalent circuit models of GaN HEMTs are discussed by Mi et al,7 Chen et al,8 and Wang et al,9 respectively. The modeling of emerging devices is also presented by Chen et al10 for GaN-on-diamond HEMTs and Zhang et al11 for AlGaN/GaN fin-shaped HEMTs, which may be interesting for next generation devices. Accurate modeling of transistors' noise performance is important for low-noise applications. Jarndal et al12 developed a noise model for GaN HEMTs and Caddemi et al13 reported an improved scalable parameter extraction method for GaAs HEMTs. CMOS transistor is very competitive in millimeter-wave and THz circuits due to its mature process. Bashir et al14 presented a scalable small-signal model for RF CMOS transistors. Besides compact models, progress of artificial neural network-based modeling for small-signal models is presented by Marinković et al.15 Accurate de-embedding for CMOS transistors reported by Wang et al16 and Xie and Xu17 can be found helpful for active device modeling. Different from semiconductor devices, electric vacuum devices have high-power, high-efficiency, and high-gain characteristics in the millimeter-wave and THz frequency bands. Guo et al18 reported the modeling, simulation, and fabrication of an electron optic system for high-power 105 GHz gyrotrons. Chen et al19 proposed a multisection coupling cavity for high-frequency extended interaction klystrons. All of these efforts toward active device modeling can provide a deeper understanding of the relationship between models and circuits as demonstrated in the paper by Chen and Xu,20 which theoretically explores the impedance of transistor for circuit design based on a large-signal model. Besides active device modeling, modeling and design techniques of passive devices are also covered in this special issue. In the design of InP HBT-based circuits, Cui et al21 reported a broadband subharmonic mixer in the frequency range of 199 to 238 GHz by combining the diode SPICE model and 3D full-wave electromagnetic (EM) model. Li et al22 proposed an EM simulation-based method for InP HBTs model parameter extraction, which is applied to 180 to 240 GHz amplifier design. Passive device models in CMOS technology are quite complicated due to the small area. Zhang et al23 presented an improved wideband single-π model for on-chip spiral inductors based on independent test ground PADs. A different group by Zhang et al24 reported a balun design algorithm based on compensation matching capacitors and their active S parameters. Using CMOS technology, Chen et al25 designed a millimeter-wave up-conversion mixer by using a two-path transconductance stage and Yu et al26 reported a linear V-band low-noise amplifier by employing a Gm-boosting technique using full-wave high-frequency structure simulator. Other technologies like GaAs MMIC, hybrid integrated circuits, and electric vacuum devices are also presented by Li et al,27 Yang et al,28 and Wang et al,29 respectively. In addition to active and passive device and circuit design, devices for emerging applications like THz sensors and THz absorbers are reported by Cheng et al30 and Aghaee and Orouji.31 Niu et al32 presented a 220 GHz signal monitoring system, where the front end of the receiver consists of a subharmonic mixer and a tripler based on Schottky diodes. The proposed method can be useful for high-performance millimeter-wave and THz systems. At the end, I would like to take this opportunity to thank all the authors for submitting their papers to this special issue and all the reviewers for dedicating their time to help improve the quality of the papers.