Objective Perfusion single photon emission computed tomography (SPECT) and 18F fluorodeoxyglucose positron emission tomography (FDG‐PET) both have clinical utility for the differential diagnosis of dementia. Although PET is often viewed by some as more accurate and therefore preferential, the extent to which published evidence supports this is not clear. The aim of this review was to address the question by reviewing studies of SPECT and PET imaging in dementia diagnosis, with a particular focus on all published head‐to‐head studies. Design A MEDLINE search was carried out using the following keywords: “PET” and “SPECT” and “dementia” or “Mild Cognitive Impairment,” together with “alzheimers” or “DLB” or “lewy body” or “frontotemporal” or “FTD” or “Picks.” Articles were included up to February 2013, limited to human studies and in English language. Results Published studies of SPECT accuracy show that it is a useful tool for differential diagnosis, with sensitivities of 65–85% for diagnosing Alzheimer's disease (AD) and specificities (for other neurodegenerative dementias) of 72–87%. PET studies generally report higher accuracy, with sensitivities of 75–99% for AD and specificities of 71–93%. However, there have been few direct head‐to‐head comparisons, with some indicating SPECT and PET to be equally useful in dementia diagnosis and others favouring PET. Many of these studies are limited with respect to numbers and methodically with poorly matched control groups. Conclusions Overall, although studies suggest superiority of PET over SPECT, the evidence base for this is actually quite limited. We suggest that further direct comparative studies, including health economic and patient preference evaluations, are needed to help direct future service provision. Copyright © 2013 John Wiley & Sons, Ltd.