Through the use of in vitro selection techniques, a number of RNA aptamers have been selected for their ability to bind ligands with high affinity and specificity. The three-dimensional solution structures of a number of these complexes have been solved within the last 4 years. This review focuses on the structural characterization of the RNA aptamers bound to the cofactors FMN and AMP, the amino acids arginine and citrulline, the drug theophylline and the aminoglycoside antibiotic tobramycin in solution. Analysis of the structural features of these complexes allows the identification of molecular themes in RNA aptamer structure, recognition and discrimination.