This book highlights the application of active array antennas in high-resolution microwave imaging radar systems. It introduces the basic principles, analytical methods, and performance parameters of active array antennas to achieve low profile, high efficiency, and lightweight. The book systematically elaborates the architecture, analysis, and engineering practice to achieve wideband, multi-band, multi-polarization, and common aperture in active array antennas. It explores hotspot technologies of digital array antennas, microwave photonic array antennas, and active packaging antennas. By presenting over 300 illustrations and diagrams, including schematic diagrams, block diagrams, relation diagrams, and breakdown drawings, the book enables a thorough understanding of the antenna array microsystem as the advanced phase of active array antennas and the direction of future R&D.  The book is a good reference source for researchers and engineers interested in the engineering and implementation of microwave imaging radar systems and antenna technology.