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

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