The increasing demand for high bit rate wireless communication prompts people resorting to the mmWave spectrum. A single chip radio becomes possible for the small wavelength of millimeter EM waves. The antenna plays an important role in a radio because its independent properties can affect the radio as a whole. Nowadays, mmWave antenna designs are shifting from the design of a conventional discrete device to antenna-on-chip (AoC) and antenna-in-package (AiP). The AiP is an independently designed device from the integrated circuit (IC), which leads to a wide selection of antenna prototypes. Various antennas can be utilized as mmWave AiPs, among which the grid array antenna (GAA) is a capable candidate. The book is devoted to provide a physical insight into the characteristics of mmWave GAAs, develop a synthesis method for mmWave GAA design, and provide solutions for mmWave AiP applications using GAA as the antenna prototype.