Incoherently illuminated or self-luminous objects are composed of many independent object points. Any two points from the object are spatially incoherent whereas the two beams from the same object point are spatially coherent. Incoherent digital holography is a technology of recording holograms of incoherent objects and reconstructing three-dimensional information of the original object. The hologram of a point object can be formed based on spatial self-coherence of the beams by suitable optical beam-splitting technique and the hologram of an extended object is the superimposing of all point-holograms incoherently. The investigations on incoherent digital holography have verified that this technique has great potential in non-scanning fluorescent microscopy, incoherent color holography, and incoherent adaptive optics. The basic principle of incoherent digital holography is firstly demonstrated and some typical incoherent holographic imaging systems are summarized. The key imaging characteristics and research progress are analyzed and discussed based on self-reference incoherent holography.