Few topics in astronomy initiate such vigorous discussion as whether or not the initial mass function (IMF) of stars is universal, or instead sensitive to the initial conditions of star formation. The distinction is of critical importance: the IMF influences most of the observable properties of stellar populations and galaxies, and detecting variations in the IMF could provide deep insights into the process by which stars form. In this contribution, we take a critical look at the case for IMF variations, with a view towards whether other explanations are sufficient given the evidence. Studies of the field, local young clusters and associations, and old globular clusters suggest that the vast majority were drawn from a "universal" IMF. Observations of resolved stellar populations and the integrated properties of most galaxies are also consistent with a "universal IMF", suggesting no gross variations in the IMF over much of cosmic time. Here we focus on 1) nearby star-forming regions, where individual stars can be resolved to give a complete view of the IMF, 2) star-burst environments, in particular super-star clusters which are some of the most extreme objects in the universe and 3) nearby stellar systems (e.g. globular clusters and dwarf spheroidal galaxies) that formed at high redshift and can be studied in extreme detail (i.e. near-field cosmology).
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