Dementia and Alzheimer's disease (AD) are highly debilitating conditions that afflict millions of elderly persons. In recent decades, biological evidence has implicated chronic stress in the etiology of these conditions. As a result, the relationship between widowhood, one of the most stressful life events, and dementia and AD has also received attention. However, studies are mixed regarding this association, and few have investigated whether this relationship is moderated by the context surrounding widowhood. This study extends this literature by investigating whether widowhood increases risk for dementia and AD and whether this risk is moderated by contextual factors including age at widowhood, remarriage after widowhood, manner of death, number of dependent and adult children at the time of widowhood, gender, presence of epsilon 4 allele of apolipoprotein E (APOE), and history of depression and antidepressant use. To do this, this investigation utilized data from the Cache County Memory Study (CCMS), a large population-based epidemiological study of dementia and AD, and the Utah Population Database (UPDB), one of the world's foremost linked genealogical databases. In Cox regression analyses that modeled time to onset of dementia and AD, gender was found to moderate the relationship between incident widowhood and dementia (HR = 1.74, 95% CI: 0.97-3.10), in that widowhood trended towards decreased risk among men (HR =0.72, CI: 0.45-1.16) but increased risk among women (HR = 1.21, CI: 0.83-1.75) in stratified models. In addition, history of depression and antidepressant use moderated the association between incident widowhood and dementia (HR = 2.63, 95% CI: 1.26-5.50) and AD (HR = 1.68, 95% CI: 1.11-2.53), in that widowhood was associated with decreased risk for dementia and AD among the never depressed (HR = 0.66, CI: 0.42-1.02 and HR = 0.54, CI: 0.31-0.92, respectively), a trend towards increased risk for AD among those with a history of antidepressant use but no depression (HR = 1.80, CI: 0.86-3.75), and with increased risk for dementia and AD among those with a history of both (HR = 1.93, CI: 0.98-3.81 and HR = 1.89, CI: 0.80-4.43). These findings advance clinical and scientific knowledge concerning the effects of widowhood on risk for dementia and AD, and underscore the importance of context in understanding this relationship.