Eukaryotic cells contain numerous RNA quality-control systems that are important for shaping the transcriptome of eukaryotic cells. These systems not only prevent accumulation of nonfunctional RNAs but also regulate normal mRNAs, repress viral and parasitic RNAs, and potentially contribute to the evolution of new RNAs and hence proteins. These quality-control circuits can be viewed as a series of kinetic competitions between steps in normal RNA biogenesis or function and RNA degradation pathways. These RNA quality-control circuits depend on specific adaptor proteins that target aberrant RNAs for degradation as well as the coupling of individual steps in mRNA biogenesis and function.