In this volume of the series Human Brain Function: Assessment and Rehabilitation we cover the area of how brain function is assessed with behavioral or neuropsycholog­ ical instruments. These assessments are typically conducted by clinical neuropsy­ chologists or behavioral neurologists, and so we made an effort to present the somewhat differing approaches to these two related disciplines. Clinical neuropsy­ chologists are psychologists who typically utilize standardized tests, while behav­ ioral neurologists are physicians who generally assess brain function as part of the clinical neurological evaluation. Both approaches have much to offer. The basic assumption of neuropsychological assessment is that the brain is the organ of behavior, and therefore, the condition of the brain may be evaluated with behavioral measures. Neuropsychological tests are those measures found by re­ search to be particularly sensitive to alterations in brain function. An adequate neuropsychological test is a procedure that can be related to some objective mea­ sure of alteration in brain function. Over the years, these objective measures have changed, but generally involve documentation through direct observation of brain tissue, or through histological, pathological, neuroimaging, or other laboratory procedures. The methods described in the first two volumes of this series describe the neuroimaging procedures that are often used in the validation of neuropsycho­ logical tests.