The Neuropsychiatric Inventory Questionnaire (NPI-Q) is a collateral-rated measure of behavioral and psychological symptoms (BPS) commonly found in dementia. The construct of BPS in dementia is complicated by the fact that different diseases can result in identical symptoms or syndromes (i.e., equifinality), and the same disease can result in different syndromes (i.e., multifinality). Thus, BPS may emerge in individuals with various types of health conditions other than dementia. It is important to assess BPS regardless of the underlying pathology given its ubiquitous nature in healthcare settings and association with cognitive and functional decline as well as reduced quality of life. The NPI-Q provides a reliable way to assess BPS in individuals who may be unable to accurately report their own symptoms (e.g., alexithymia, anosognosia, disorientation). Although some aspects of the NPI-Q’s validity (e.g., structural validity, convergent validity) have been demonstrated in various clinical populations, others remain underexplored (e.g., criterion validity). This paucity of research may be partially attributed to the complex nature of BPS and lack of consensus for a criterion. Ultimately, the NPI-Q’s strength is also a limitation, depending on the context. Its strength is that it incorporates multiple questions within single items, allowing for quick and broad sampling of multiple BPS. However, this approach also limits the ability to assess granular aspects of BPS.