The RNA pseudoknot is an essential folding motif found in many classes of ribonucleic acids. Pseudoknot structures are involved in translational frameshifting, and they are found in the catalytic RNA component of RnaseP, and the cleavage site of the Hepatitis Delta Virus. Recent evidence shows that pseudoknots are potent inhibitors of the reverse transcriptase of HIV. To date, no crystal structrure is available to elucidate the detailed molecular geometry and tertiary interactions of the pseudoknot.