Significant advances are being made in our understanding of basic pathophyiological and biochemical mechanisms that cause Huntington's disease (HD). There is increasing reason to believe that pathologic alterations occur in the brain for years before symptoms manifest. The "classic" hallmark of neuropathology in HD is selective neurodegeneration in which vulnerable populations of neurons degenerate while less vulnerable populations are spared. While the earliest and most striking neuropathologic changes have been found in the neostriatum, neuronal loss has been identified in many other regions of the brain. We report topologically selective, early, and progressive changes in the cortex, striatum, extrastriatal brain structures, and white matter throughout the spectrum of disease. Our growing understanding of HD underscores the reality that points to the complexity of HD. A single, well-defined, genetic mutation causes a cascade of events whose final result is an aggregate insult of the homeostatic process. We explore possible explanations for the selective vulnerability of the brain in HD. The ultimate goal in HD is to develop disease-modifying therapies that will prevent the onset of clinical symptoms in those individuals who are at risk and slow the progression of symptoms in those individuals already affected with symptoms. Understanding changes in brain morphometry and their relationship to clinical symptoms may provide important and new insights into basic pathophysiological mechanisms at play in the disease.