Terms to describe the behavioral and psychological symptoms commonly seen in dementia, including “Behavioral and Psychological Symptoms of Dementia” (BPSD), “non-cognitive symptoms,” and “neuropsychiatric symptoms,” were introduced in the 1980s and 1990s to draw attention to the heterogeneous group of symptoms that, distinct from cognitive deficits, are commonly seen in dementia and cause significant distress to patients and carers (Reisberg et al., 1987; Cummings et al., 1994; Allen and Burns, 1995; Finkel et al., 1996). BPSD include a wide range of affective, psychotic, and hyperactivity symptoms, and studies include different combinations of symptoms. These symptoms are also often studied individually outside the context of BPSD in the older population with or without cognitive impairment. Depression is most frequently studied, particularly in the older population without dementia. The relationship between dementia and depression in older people and the courses of the two disorders have been an important research topic for around 70 years (Roth, 1955). Traditionally, depression in the population without dementia is considered as a phenomenon distinct from BPSD, and thus BPSD research and depression research in older people have been largely separate endeavors. BPSD research typically studies sub-threshold depression as well as other behavioral and psychological symptoms in dementia and those with more severe depression are often excluded, along with people without dementia. Depression research typically includes major or minor depression in all older adults but often excludes people with cognitive impairment or other psychiatric problems. However, there are several areas where these two research fields may overlap. Depression in a cognitively healthy older person may indicate early dementia, and depression may be a risk factor for dementia (Ownby et al., 2006). Continuities in depression are seen “pre” and “post” dementia diagnosis, and common biological and psychosocial risk factors for depression may exist among the cognitively intact and cognitively impaired older populations. Some sense of the many permutations in the literature is essential to understanding the meaning of research findings reported by different studies and how they may be compared. Here we explore how depression and/or other BPSD have been studied using different inclusion and exclusion criteria and on populations with different levels of cognitive impairment. We have previously published a review of systematic reviews on behavioral and psychological symptoms in the older population (van der Linde et al., 2012), and we identified three high-quality reviews investigating the prevalence of depression and/or BPSD in different populations (Verkaik et al., 2007; Monastero et al., 2009; Luppa et al., 2010) as well as two additional reviews on the associations of depression (Jorm et al., 1991; Christensen et al., 1997).