| The ghost imaging (GI),which is also named correlated imaging,is currently a hot topic in quantum optics. Based on the correlation of entanglement or field fluctuations,a clear image of a non localized object is obtained,at the idler path,by harnessing the intensity correlation between the signal beam and the idle beam. GI has huge values and extensive usages in optically harsh (difficult to be reached) or noisy environments. After more than two decades of development,GI has found its application in military,LIDAR,optical encryption,so on,and their future research on pathological analysis,material research and life sciences.By reviewing the development history of GI,the priciple of ghost imaging is introduced,the factors affecting the quality of ghost imaging are analyzed,and the progress of speckle generation,reconstruction and optimization of GI system are described. Finally, the applications of GI in the optical encryption and the edge detection are demonstrated.