Abstract: The emergence of ultra-intense, ultrafast, and coherent X-ray free electron lasers (FELs) offers opportunities for ultrafast time-scale studies and ultra-small structure determination,making it possible to acquire images and movies of single-molecules and single-particles at atomic resolution. With the completion and use of the FLASH, FERMI, LCLS, and SACLA facilities, X-ray FELs have entered a stage of rapid development, and a series of cutting-edge research results in the fields of physics, chemistry, biology, and materials science are emerging. To realize single particle imaging, research institutions are collaborating worldwide to overcome the technical barriers in experimental techniques, X-ray instrumentation and data analysis. They aim to achieve imaging of nanoparticles, bacteria, cells, viruses, clusters and biological macromolecules on the atomic scale. In this review we will give a brief introduction to the background, scientific significance, objectives, research roadmap, current status, and prospective future development of single particle imaging.