People aged 90 and older are the fastest growing age group in most parts of the world. Since the prevalence of dementia has been shown to increase exponentially after the age of 65, there is an acceptance that the oldest old population has a high burden of dementia; however, there is a lack of consensus on how best to diagnose dementia in this population. This review summarizes the various approaches to diagnosing dementia and the prevalence and incidence rates of dementia that have been reported. We also summarize the literature on cognitive and functional performance and biomarkers for dementia and discuss the limitations to interpretation of these data. Finally, we make recommendations for both researchers and clinicians who intend to diagnose dementia in the oldest old population.