The fifth edition of Clinical Neurology, like its predecessors, offers a problem-oriented approach to neurology based on the authors' experience in teaching medical students and house staff at the University of California, San Francisco. Chapters are organized according to problems such as headache, seizures, stroke, and coma, because these are the conditions for which patients usually seek medical care. Careful history taking and neurologic examination are emphasized, as these remain the cornerstones of neurologic diagnosis, even in an era of technologic diagnostic advances. The need to update this book arises from two main sources: rapid expansion of knowledge about the molecular basis of neurologic diseases and recent innovations in the treatment of disorders such as headache, epilepsy, stroke, Parkinson's disease, and multiple sclerosis. Accordingly, increased prominence has been given to molecular mechanisms of diseases—for example, Alzheimer's disease and the polyglutamine disorders, including Huntington's disease. Sections on treatment have been updated and expanded to reflect the introduction of new therapies for neurological disorders. The summary tables of therapeutic drugs and genetic disorders inside the front and back covers, which were introduced in the last edition, have been revised to maintain currency. Key Concepts is a new feature that has been introduced in this issue. In the beginning of each chapter, some of the major concepts are presented with numbered icons. These same numbered icons appear within the text to indicate where these specific points are discussed in the chapter. We thank our colleagues, who have contributed their expert advice to the preparation of this new edition of Clinical Neurology, especially Lydia Bayne, Megan Burns, Chadwick Christine, Paul Garcia, Alisa Gean, Cheryl Jay, Catherine Lomen-Hoerth, Neil Raskin, Tom Shults, and Norman So. The staff at McGraw-Hill have been enormously helpful in moving this book through editing and production. We hope our efforts will help to demystify clinical neurology for students and practitioners and contribute to providing patients better and more focused diagnosis and treatment.