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We present a review on the determination of the primordial helium abundanceYp, based on the study of hydrogen and helium recombination lines inextragalactic HII regions. We also discuss the observational determinations ofthe increase of helium to the increase of oxygen by mass Delta Y / Delta O, andcompare them with predictions based on models of galactic chemical evolution.

Measurements of 4He in Metal-Poor Extragalactic HII Regions: the Primordial Helium Abundance and the Delta Y / Delta O Ratio