Huntington's Disease (HD) is an autosomal dominant disease manifested in a triad of cognitive, psychiatric, and motor signs and symptoms. HD is caused by a triplet repeat (CAG)expansion in the gene Huntingtin (HTT). This disease has classically been conceptualized as a neurodegenerative disease. However, recent evidence suggests that abnormal brain development may play an important role in the etiology of HD. Huntingtin (HTT)is expressed during development and through life. In animal studies, the HTT gene has been shown to be vital for brain development. This suggests that a mutant form of HTT (gene-expanded or CAG repeats of 40 and above) would affect normal brain development. In addition, studies in adults who are gene-expanded for HD, but have not yet manifested the illness, (pre-HD subjects) have significant changes in the structure of their brain, even up to 20 years before onset of clinical diagnosis. How far back these changes are evident is unknown. One possibility is that these brain changes are present throughout life, due to changes in brain development,though initially associated with only subtle functional abnormalities. In an effort to better understand the developmental aspects of this brain disease, the current study proposes to evaluate brain structure and function in children, adolescents, and young adults (ages 6-30) who are at risk for developing HD - those who have a parent or grandparent with HD. Brain structure will be evaluating using Magnetic Resonance Imaging (MRI) with quantitative measures of the entire brain, cerebral cortex, as well as white matter integrity via Diffusion Tensor Imaging. Brain function will be assessed by cognitive tests, behavioral assessment, and physical and neurologic evaluation. Subjects that are gene-expanded (GE) will be compared to subjects who are gene non-expanded (GNE). Changes in brain structure and/or function in the GE group compared to the GNE group would lend support to the notion that this disease has an important developmental component.