Heterogeneous nuclear r ibonuc leopro te ins (hnRNPs) associate with RNA polymerase II transcripts immediately following the initiation of transcription to form hnRNP complexes. These structures are the functional assemblies within which hnRNAs and pre-mRNAs exist in the nucleus. The hnRNP descriptive has been used to refer to the totality of proteins whose primary binding site is hnRNA, and/or its associated proteins, as well as exclusively those six proteins associated with nuclease-resistant particles isolated from sucrose gradients. For this review, all of the proteins which have been demonstrated to interact with hnRNAs/pre-mRNAs will be covered. In addition, only those studies reported since the last European RNP meeting in 1987, will be discussed; extensive reviews of previous literature are available (Chung & Wooley, 1986;Dreyfuss, 1986).