The most commonly encountered clinical conditions presenting with cognitive failure (CF) are delirium, dementia and amnestic disorders. Of these, delirium is probably the most prevalent in palliative care, and it is potentially reversible. Thus, improvement in diagnostics seems warranted. The objectives of this review were to examine the methods for assessment of CF and delirium in palliative care. Twenty-two studies were reviewed: 64% were published in 2000 or later. Twelve reports focused on delirium, six on CF, while the remaining four assessed confusion (2), hallucinations and general psychological morbidity. Median sample size was 100 (20393). Ten different instruments were used: The Mini Mental State Exam was used in 13 studies. Five studies were validation reports of new or existing instruments. The term CF is an imprecise description of a loss in one or more of the cognitive functions. The interchangeable use of CF as a description of specific diagnoses should be avoided, as this contributes to prevalence rates that are not representative. Assessment tools that discriminate between the different diagnostic entities presenting with CF should be used in future studies.