The gas and dust in giant molecular clouds gradually come together under the influence of gravity to form stars. Precisely how this occurs, however, is incompletely understood. The mass of a star, for example, is by far the most important factor constraining its future evolution, but astronomers do not clearly understand what determines the exact mass of a newly forming star. One aspect of this problem is simply knowing how many stars of each size there are, that is, knowing the distribution of stellar masses in a large cluster of stars. The initial mass function (IMF) describes this distribution, and is currently based on an average from observations of stars in our Milky Way.