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; ataxias; 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.