The proportion of persons aged 85 and over, the so-called "oldest old", is increasing dramatically worldwide. While a quarter of this population is affected by dementia, little is known about the specific features of cognitive functioning in the oldest old. In the presence of clinical specificities such as numerous comorbidities, multi-medication and visual and/or auditory loss, which are very frequent in extreme old age, neuropsychological assessment can be particularly challenging. This article presents an overview of the epidemiology of cognitive functioning in the oldest old, and discusses the issues regarding neuropsychological assessment and dementia in this specific elderly population.