The most important characteristic of star, which determines its properties and evolutionary path, is its mass at birth. Observations show that the most common birth mass is slightly smaller than the mass of the Sun, and that masses either much lower or much higher than this value are increasingly rare. This characteristic stellar mass appears to be nearly invariant across a huge range of environments within and beyond our Galaxy, and seems to have changed little over most of cosmic time.

Explaining its origin and universality is one of the oldest problems in theoretical astrophysics, but a fully successful theory eludes us even today. In this talk, I discuss both the history of this problem and recent progress suggesting that we may be within reach of a solution.