Specific interactions between RNAs and proteins are fundamental to many cellular processes, including the assembly and function of ribonucleoprotein particles (RNPs), such as ribosomes and spliceosomes and the post-transcriptional regulation of gene expression. Among the complexes studied to date are small RNAs bound to individual amino acids, tRNAs and tRNA fragments bound to their cognate aminoacyl-tRNA synthetases, and a variety of proteins bound to RNA single strands, hairpins, irregular helices, and tertiary structures stabilized by bound cations. Several proteins use a beta-sheet surface to bind RNAs, and others insert an alpha-helix into the widened major groove of a non-canonical RNA helix. Distortion or rearrangement of the RNA structure by bound protein is a common theme. The structural details of protein-RNA complexes are being resolved by nuclear magnetic resonance (NMR) and X-ray crystallography, but thorough thermodynamic analyses of recognition mechanisms have yet to be performed.