Neuropsychological assessment has a distinct role in the detection and monitoring of cognitive and functional changes associated with dementing illness. Molecular, structural and functional neuroimaging studies have advanced our understanding of the anatomy and physiology underlying neurodegenerative disease; however, the overlap in pathological features of different dementia-associated diseases limits the information that can be obtained by these methods. Incorporation of information obtained from multiple sources can help to increase diagnostic and prognostic accuracy. Neuropsychological test findings provide unique value as biomarkers of dementia, as differentiators of disease topography and in the estimation of disease risk and trajectory. However, psychometric test properties--such as construct validity, stability and the use of appropriate norms--must be understood, because they influence both the application of neuropsychological tests and the interpretation of their results. Finally, measurement of cognitive strengths and weaknesses in patients at risk of dementia can be helpful to predict changes in functional abilities, design appropriate and effective interventions, and assist family and health-care providers in the planning of the patient's future care needs. This Review describes the key characteristics of neuropsychological testing in the assessment of patients at risk of dementia.