Neurodegenerative disorders, such as Huntington's disease, spinocerebellar ataxias, Parkinson's disease or Alzheimer's disease, manifest in adult age with insidiously developing, slowly progressing symptoms. At this stage, most patients consult a doctor, and a definite diagnosis can be made. It is, however, well established that the manifest disease is preceded by a presymptomatic disease stage that may last for years. A striking example is Parkinson's disease, in which more than half of the dopaminergic neurons of the substantia nigra are lost before motor symptoms appear. Studies of the presymptomatic stage of neurodegenerative disorders are pivotal for an advanced understanding of these disorders and the development of preventive strategies aimed at postponing the clinical onset of these disorders. It is therefore important to identify the earliest and most sensitive clinical signs and biological markers that herald the onset of the illness. Furthermore, studies of presymptomatic disease stages are important because they may help to unravel compensatory mechanisms responsible for apparently normal brain function despite ongoing neurodegeneration.