Until recently, retention of introns in mature mRNAs has been regarded as a consequence of mis‐splicing. Intron‐retaining transcripts are thought to be non‐functional because they are readily degraded by nonsense‐mediated decay. However, recent advances in next‐generation sequencing technologies have enabled the detection of numerous transcripts that retain introns. As we review herein, intron‐retaining mRNAs play an essential conserved role in normal physiology and an emergent role in diverse diseases. Intron retention should no longer be overlooked as a key mechanism that (...) independently reduces gene expression in normal biology. Exploring its contribution to the development and/or maintenance of diseases is of increasing importance. (shrink)