Since Roentgen discovered x‐ray and performed the first x‐ray imaging over 100 years ago, x‐ray medical imaging has always been based on the biological tissue's differences in x‐ray attenuation. However, x‐ray‐tissue interaction causes x‐ray phase shifts as well, and the phase‐shift differences between different tissues are about one thousand times larger than their attenuation differences. The phase‐sensitive x‐ray imaging hence allows not only visualizing the tissues with very low attenuation‐contrast, but also quantifying tissue's projected electron densities by means of the phase retrieval. In this presentation we first introduce the concept of spatial coherence of x‐ray wavefield and elucidate the mechanism of x‐ray phase‐contrast formation. We then review recent progress in the in‐line phase—sensitive x‐ray imaging. Educational Objective: 1. Elucidate the mechanism of x‐ray phase—contrast formation. 2. Review recent progress in the in‐line phase—sensitive x‐ray imaging. © 2007, American Association of Physicists in Medicine. All rights reserved.