With the development of a high-speed wireless technology, low-temperature co-fired ceramics (LTCC) technology becomes popular. The high-speed, low-delay and high-reliable wireless transmission could be achieved by using LTCC substrates with a low-permittivity (K), a low-loss and a stable-temperature of resonant frequency, as one of functional materials. This review introduced the current situation of commercial substrate materials and alternative LTCCs reported, i.e., ceramic/glass, oxide-assisted sintering, fluoride-assisted sintering and intrinsic low-sintering ceramic systems. The composition, structural characteristics, dielectric properties, coefficient of thermal expansion of low-K LTCCs, as well as the advantages and disadvantages, were also discussed. Some studies of commercial substrates were further described, and their performances in millimeter-wave application were represented. In addition, the development trend of low-K LTCCs was also proposed.