Massive stars have significant effects on the ambient interstellar medium (ISM) via either strong UV radiation or powerful stellar winds. These winds also produce an important chemical pollution. Since WRs are the chemically evolved descendants of very massive stars, their winds are expected to be helium-rich and to show the effects of the CNO cycle. Here the authors examine the available theoretical predictions for the evaluation of the chemical enrichment due to massive stars in the light of the most recent abundance determinations for a sample of galactic WR nebulae and for a group of galactic OB stars. The importance of the chemical enrichment by massive stars in low metallicity starbursts, H II galaxies, is also discussed.