This chapter focuses on the evaluation of diagnostic technologies for current neurological practice. New diagnostic tests may have important implications for everyday clinical practice, for research into pathogenic mechanisms, and sometimes even for classifications of diseases. In neurology, the advent of magnetic resonance imaging (MRI) has affected the diagnostic approach to a wide range of clinical problems in a fundamental way. Studies of technical capacity may identify high quality diagnostic tests which are not necessarily more accurate than the standard diagnostic approach in identifying subjects with a specific disease. Assessment of therapeutic impact requires recording of the diagnosis and treatment plan before the test under study is performed. The technical capacity and diagnostic accuracy of a (new) test in clinical neurology should ultimately translate in health benefits for patients through its diagnostic or therapeutic impact. Entirely different considerations apply to the use of diagnostic instruments in research settings.