Studies of RNA structural motifs at high resolution by NMR and X-ray crystallographic methods have provided many insights into the fundamental forces that give rise to the unique structural characteristics of RNA. Non-Watson-Crick purine-pyrimidine, purine-purine, and pyrimidine-pyrimidine base pairing, as well as base-phosphate and base-ribose hydrogen bonding, are important forces for folding and stabilizing RNA structures. Base stacking is as important in determining RNA conformations as hydrogen bonding interactions. With the noncanonical interactions, many single-stranded loop regions such as hairpin loops, bulge loops, and internal loops fold into well-defined secondary structures. Loop-loop and loop-helix interactions can produce tertiary structures such as pseudoknots. Also, single strands adjacent to helical regions can form tertiary contacts with base-paired nucleotides of the helices. As we learn more about the structures of the important motifs we can ask more specific questions about the mechanisms of RNA-mediated functions. Conformational flexibility rather than a specific shape of the RNA may be important for some biological reactions. However, knowledge of the structures and the ease of conformational change of the molecules involved in any process are essential for understanding and eventually controlling the process.