All studies of unresolved stellar populations rely on a proper characterization of the stellar initial mass function (IMF), i.e. the distribution of stellar masses at birth. Over the past few years, several avenues of research have suggested a systematic variation of the IMF in early-type galaxies, with a departure from the standard IMF in the most massive systems towards both an enhanced contribution from low-mass dwarves (derived from line strength constraints); and an excess of stellar M/L (from galaxy dynamics and gravitational lensing constraints). We present here some of the recent results, focusing on constraints based on spectral line strengths and the consequences derived from galactic chemical enrichment.