  The primordial abundance of 4He, Y_P, is one of the hottest themes in
present-day astronomy, mostly due to its cosmological relevance. The
disagreement between different determinations has been currently reduced to the
1-2 percent level, but these differences are still large enough to have deep
implications for Big-Bang nucleosynthesis. It is therefore crucial to estimate
precisely the uncertainties involved in the measurement of Y_P. Here, I review
the methods used in the determination of Y_P and the related uncertainties. I
also discuss some recent results, and emphasize the assumptions underlying the
differences among them.
