Fourteen neurological diseases are known to be caused by anomalous expansion of unstable trinucleotide repeats. The mechanism that links such expansions to the corresponding pathologies is still unknown. It is thought to cover a variety of mechanisms ranging from interference with nucleic acid structure and transcription to alterations in protein structure and functions. Understanding the cellular role of the proteins involved in these diseases is of primary importance to design possible therapeutical approaches. Structural biology is a powerful tool for providing a detailed description at atomic resolution of protein functions and suggesting working hypotheses which can then be tested experimentally. In this review we discuss the available structural knowledge about proteins involved in trinucleotide expansion diseases and how this may influence our current means of investigation.