This study is designed to learn more about the natural history of inherited neurological disorders and the role of heredity in their development. It will examine the genetics, symptoms, disease progression, treatment, and psychological and behavioral impact of diseases in the following categories: hereditary peripheral neuropathies; hereditary myopathies; muscular dystrophies; hereditary motor neuron disorders; mitochondrial myopathies; hereditary neurocognitive disorders; inherited neurological disorders without known diagnosis; and others. Many of these diseases, which affect the brain, spinal cord, muscles, and nerves, are rare and poorly understood. Children and adults of all ages with various inherited neurological disorders may be eligible for this study. Participants will undergo a detailed medical and family history, and a family tree will be drawn. They will also have a physical and neurological examination that may include blood test and urine tests, an EEG (brain wave recordings), psychological tests, and speech and language and rehabilitation evaluations. A blood sample or skin biopsy may be taken for genetic testing. Depending on the individual patient s symptoms, imaging tests such as X-rays, CT or MRI scans and muscle and nerve testing may also be done. Information from this study may provide a better understanding of the genetic underpinnings of these disorders, contributing to improved diagnosis, treatment, and genetic counseling, and perhaps leading to additional studies in these areas.