The stellar initial mass function (IMF), i.e. the distribution of stellar masses formed in a star formation event,  is a key ingredient to understand the formation and evolution of galaxies, and accurately measuring it is crucial. The past decade has seen enormous progress in the determination of the IMF. I will review the spectroscopic approach for measuring this fundamental property that defines so much of astrophysics and will summarise the observational measurements from extragalactic studies.