X-ray diffraction imaging (XDI) is a technique for visualizing the structures of non-crystalline particles with dimensions ranging from micrometers to submicrometers. In XDI experiments, non-crystalline particles are irradiated by X-rays with high transverse coherence. Then, the Fraunhofer diffraction pattern is recorded under the oversampling condition at a desired resolution. The electron density map of the particle projected along the direction of the incident X-ray is reconstructed by applying the phase-retrieval (PR) algorithm to the diffraction pattern alone. In this chapter, the spatial coherence of X-rays, the X-ray source necessary for XDI, and the representative algorithms used for PR are introduced.