Two generalizations can be drawn from the recent rapid progress in understanding RNA-protein interactions. First, there is a great diversity of observed protein and RNA structural motifs. Second, formation of almost every RNA-protein complex that has been characterized involves conformational changes in the protein, the RNA, or both. The role of these conformational changes in the biological function of RNA-protein complexes is not at all clear. Whether or not conformational changes are a critical feature of ribonucleoprotein complex assembly or are an unimportant mechanistic detail, the ubiquity of these changes warrants careful consideration of their implications.