We begin this article by presenting a model for gas permeation through defects in barrier coatings, which is based on a very simple geometrical approach. This model allows us to evaluate permeation through a single- or through multiple circular defects in the coating, and also in special cases of size distributions and noncircular geometries. The model agrees well with published results based on complex computer simulations, and it has proven very useful in the analysis of our permeation measurements for barrier-coated plastic films. We then present correlations between measured O-2 transmission rate (OTR) values and the number densities and size distributions of defects in SiO2 and SIN barrier coatings on polyester (PET), results which we compare with published data for aluminized PET. We also show the temperature dependence of OTR and H2O vapor transmission rate fur SiO2 and SiN coatings deposited on one and on both sides of PET films. The apparent activation energies evaluated from these measurements support the model of defect-dominated mass transport, but suggest different mechanisms for oxygen and water vapor transmission: Excellent agreement is found between measured and calculated OTR values; these, and the apparent activation energy for OTR through single or double side SiO2 and SiN coatings on PET films, confirm that the observed residual OTR is indeed controlled by the presence of (pinhole) defects in the coatings.