Reasons for the Study: The Neuropsychiatric Inventory (NPI) has been used for studies of neuropsychiatric symptoms in neurodegenerative disorders for the past 25 years. This article reviews the history of the development and application of the NPI. Main Findings: The NPI consists of 10 (or 12) items that are assayed with questions, subquestions, and ratings of frequency and severity. The NPI has been shown to be valid and reliable. The NPI has been translated into approximately 40 languages; it has 4 of versions designed for different clinical applications. The NPI studies show contrasting profiles of behavioral symptoms in different neurologic disorders. The NPI has been used in approximately 350 clinical trials. In economic studies, the NPI captures the cost of behavioral symptoms in dementias. Principle Conclusions: The NPI is a useful instrument for capturing behavioral changes in Alzheimer disease and other neurodegenerative disorders.