In eukaryotic cells, messenger RNAs are formed by extensive post-transcriptional processing of primary transcripts, assembled with a large number of proteins and processing factors in ribonucleoprotein complexes. The protein moiety of these complexes mainly constitutes a class of about 20 major polypeptides called heterogeneous nuclear ribonucleoproteins or hnRNPs. The function and the mechanism of action of hnRNPs is still not fully understood, but the identification of RNA binding domains and RNA binding specificities, and the development of new functional assays, has stimulated interest in them. In contrast to previous models that hypothesised a mere structural (histone-like) function, a more diversified and dynamic role for these proteins is now emerging. In fact, they can be viewed as a subset of the trans-acting pre-mRNA maturation factors. They might actively participate in post-transcriptional events such as regulated splicing and mRNA export. Moreover, recent data suggest an involvement of some of these proteins in molecular diseases. Here we present an overview of the most relevant properties of hnRNPs and discuss some emerging ideas on their roles.