The primordial abundance of 4He, YP, 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% 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 YP. Here, I review the methods used in the determination of YP and the related uncertainties. I also discuss some recent results, and emphasize the assumptions underlying the differences among them.