mRNA is packaged into ribonucleoprotein particles called mRNPs. A multitude of RNA-binding proteins as well as a host of associated proteins participate in the fate of mRNA from transcription and processing in the nucleus to translation and decay in the cytoplasm. Methodological innovations in cell biology and genome-wide high-throughput approaches have revealed an unexpected diversity of mRNA-associated proteins and unforeseen interconnections between mRNA-processing steps. Recent insights into mRNP formation in vivo have also highlighted the importance of mRNP packaging, which can sort RNAs on the basis of their length and determine mRNA fate through alternative mRNP assembly, processing and export pathways.