Understanding the stellar initial mass function (IMF) is a key aspect to obtain a complete picture of galaxy formation and evolution. In the last years, we have carried out a systematic census of the IMF in the unresolved stellar populations of early-type galaxies (ETGs), using optical and NIR spectroscopy from different surveys (e.g. SDSS, CALIFA) and dedicated observing programmes (OSIRIS@GTC, XSHOOTER and MUSE@VLT). I will show results on the non-universal IMF of ETGs, pointing to an excess of low-mass stars, i.e. a bottom-heavy distribution, in high-, relative to low-, mass galaxies, and current constraints on the physical driver behind observed variations in the stellar IMF.