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.
