Correlation imaging, also known as "ghost imaging", is a new imaging method. It calculates the coincidence between a detected light signal carrying the light intensity information of an object and the measurement matrix of the reference light path; the image information of the object is obtained by using the second-order intensity correlation characteristics between the two light paths. Different from traditional optical imaging, correlation imaging has the advantages of strong anti-noise ability and various imaging methods, which can reduce the influence of scattering medium on the disturbance and attenuation of optical signals. Since the first ghost imaging experiment in 1995, it has become one of the most popular research topics in classical and quantum physics. In this article, the history of ghost imaging development, its theoretical principle, and the latest research on ghost imaging applications are presented.