The stellar Initial Mass Function (IMF) can be conveniently represented as a canonical two-part power law function and is largely invariant for star formation regions evident in the Local Group of galaxies. The lack of massive stars in regions of low star formation density and other evidence imply that the IMF is not a probability density distribution function, but is instead closer to an optimal density distribution function. Binary stars and stellar dynamics have a very significant influence on the counts of low-mass stars and need to be corrected for. This chapter offers a review of how recent advancements in the measurement of the IMF from detailed star counts in different environments (e.g., young massive clusters, globular clusters, elliptical galaxies and ultrafaint dwarf galaxies), properly interpreted via detailed numerical models, indicate that the environmental conditions and metallicity both have an impact on the shape of the IMF.