Abstract: Combining the traditional radar and modern quantum information processing technology, quantum radar provides a strong advantage over the classical counterparts for detection of low-reflectivity targets embedded in a thermal-noise bath. As one promising type of quantum radar, quantum illumination exploits entangled photons and quantum correlation measurement to detect whether the target is present or not. Here, we firstly introduce the concept and classification of quantum radar, then analyze the fundamental and key technology of double-photon entangled state quantum illumination and Gaussian state quantum illumination. Comparing two kinds of quantum illumination radar with the classical detection methods, we find quantum illumination radar has advantages in terms of signal noise ratio, accuracy, robustness and stealthiness. In the end, we discuss open questions and future development trends of quantum illumination radar.