The pathological expansion of unstable trinucleotide repeats currently is known to cause 14 neurological diseases. Over the past several years, researchers have concentrated on the challenging task of identifying the mechanism by which the expanded trinucleotide repeat leads to abnormal cellular function. As a consequence, the trinucleotide repeat field has grown dramatically since the initial discovery of dynamic mutations less than a decade ago. Trinucleotide repeat expansions may prove to cause pathology through a variety of mechanisms including interference with DNA structure, transcription, RNA-protein interaction and altered protein conformations/interactions. The goal of this review is to provide a brief description of the genes harboring expanded repeats, coupled with new insights into the molecular pathways most likely to be disrupted by these expansions. Data from studies of patient material, cell culture and animal models demonstrate the complexity of the pathogenic mechanisms in each of the diseases.