The KH motif has recently been identified in single or multiple copies in a number of RNA associated proteins. Here we review the current knowledge accumulated about the sequence, structure, and functions of the KH. The multidomain architecture of most of the KH-containing proteins inspired an approach based on the production of peptides spanning the sequence of an isolated KH motif. Correct identification of the minimal length necessary for producing a folded peptide has had a number of important consequences for interpreting functional data. The presence of the KH motifs in fmr1, the protein responsible for the fragile X syndrome, and their possible role in the fmr1 functions are also discussed.