2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/122542We present a review on the determination of the primordial helium abundance, Yp, based on the study of hydrogen and helium recombination lines in extragalactic H II 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.8 pages, 4 figures. ASP LaTeX. Proceedings of From Stars to Galaxies: Building the pieces to build up the Universe (Venice, October 16-20, 2006). Updated reference list and slight numerical changes with respect to the first versionAstrophysicstext