Antenna-in-package (AiP) is an antenna that is realized on the package of the driving circuit. Low-temperature co-fired ceramic (LTCC) is one of the mainstream technologies for AiP designs. This chapter focuses on AiP designs in LTCC technology. It discusses LTCC technology, before moving on to details of AiP design. LTCC technology is highly suitable for substrate-integrated waveguide-based antennas. This is because LTCC is a multilayered technology in which conductive vias are an integral part of the fabrication process. One of the main challenges blocking the widespread LTCC-based AiPs use is the improvement of LTCC fabrication resolution and repeatability. LTCC technology can use low-loss conductors for the metallization steps because the multilayered LTCC tapes are laminated and co-fired at relatively low temperatures.