Huntington's disease (HD) is a fatal, genetically based brain disorder in which there is progressive neurodegeneration leading to motor, cognitive and psychiatric symptoms. The trinucleotide repeat mutation involved is common to many other brain diseases, and may therefore involve similar mechanisms of pathogenesis. We are beginning to understand how a CAG repeat expansion, encoding an expanded polyglutamine tract, induces progressive deficits in intra- and inter-cellular signaling, and subsequent disease symptoms. This review focuses on our current knowledge of molecular mechanisms of pathogenesis in HD and the use of this information to identify potential therapeutic targets and screen drugs in various transgenic models.