Measuring the polarization of the light coming from celestial sources provides a powerful tool to investigate their internal structure, as well as to study the physics at work in the emission processes. However, due to the technical challenges it presents, X-ray polarimetry is still quite undeveloped compared to other branches of astronomy, with the only significant measurement ever performed dating back to more than 40 years ago. The Imaging X-ray Polarimetry Explorer (IXPE), the next mission of the NASA SMall EXplorer program, is aimed at curtailing such a gap. Scheduled for launch in 2021, it will observe for the first time the polarization in the soft X-ray band (2–10 keV) of dozens of astrophysical objects such as black holes, active galactic nuclei, supernova remnants, magnetars and pulsar wind nebulae. At the very heart of the concept of IXPE lays the latest generation of a technology developed by INFN over the last twenty years, the Gas Pixel Detector (GPD). It is an innovative gas detector based on the photo-electric effect, which is able to image with high resolution the short tracks produced by photo-electrons of few keV in order to extract the relevant observables for imaging, spectroscopy and polarimetry. I will describe the development and design of the Gas Pixel Detector and discuss its most critical capabilities in relation to the scientific program of the IXPE mission.