The combination of large effective area X-ray optics with a new generation of more sensitive detectors has made a X-ray polarimeter a viable option for inclusion in future X-ray missions, more than 30 years after the pioneering experiments in the '70 which led to the discovery of the polarization of the X-ray emission of the Crab Nebula. In this seminar the main scientific motivations for a X-ray polarimeter will be discussed, showing that most classes of X-ray sources will strongly benefit from polarimetric observations. The observational perspectives will be also briefly discussed.