'Chemical abundances in H II regions and their implications -- Retrospective on: M. Peimbert & R. Costero, Boletin de los Observatorios de Tonantzintla y Tacubaya, Vol. 5, 3, 1969'

We present a review about the relevance of the paper by Peimbert and Costero (1969), on the chemical abundance determinations of H II regions. We analize the observational evidence in favor of the presence of temperature variations inside gaseous nebulae. We make a brief mention of the methods used to estimate the contribution of the unobserved ions to the total chemical abundances.