The mechanisms responsible for the proliferation and subsequent stabilization of introns within the eukaryotic lineage have remained elusive. In the early stages of eukaryotic evolution, most introns may have been mildly deleterious at the time of insertion, but enough of them eventually acquired integral roles in transcript processing that few eukaryotic species can any longer survive without them. We suggest that the proliferation of spliceosomal introns was facilitated by the evolution of nonsense-mediated decay, an ancient and (in many cases) intron-dependent mechanism for eliminating aberrant mRNA molecules resulting from errors in transcription and splicing and from mutations at the DNA level. The spatial distribution of introns, as revealed by whole-genome analysis, is consistent with expectations for a model in which maximum protective coverage of a gene stochastically evolves over time.