Overview: Selected as an Open Time Key Programme for Herschel in cycle 1, the Herschel infrared Galactic Plane Survey (Hi-GAL) maps the inner Milky Way galaxy in 5 bands between 60 and 600 microns with diffraction limited spatial resolution. Hi-GAL embodies the optimum combination of Herschel wavelength coverage, sensitivity, mapping strategy and speed to deliver a census of temperature, luminosity, mass and spectral energy distributions of star forming regions and cold ISM structures in all environments of the Galactic ecosystem. These measurements are provided in a single and homogeneous dataset, at unprecedented resolutions, and at all scales from massive objects in proto-clusters to the full spiral arm.