Photometric stereo is a 3D-reconstruction technique which uses n images of a scene taken under different lightings. The case n=1 is the well-known shape-from-shading problem, which is ill-posed. Since the case n>1 is much better-posed, it has become very popular in the last years. In this talk, I will first recall the main features of photometric stereo. Then, I will enumerate six open problems related to this technique. Finally, I will focus on one of these open problems.