We are excited to introduce the NHS Atlas of Variation in Diagnostic Services, the latest publication in the series of impressive NHS Atlases, which have highlighted variation in the provision of healthcare services. Diagnostic services are of great importance in the NHS because, when used correctly, they support or rule out potential diagnoses, and underpin the effective and efficient management of patient pathways. Unwarranted variation in the rates of diagnostic testing is of the utmost relevance to individual patients with the over-use, as well as under-use, of diagnostic tests being potentially serious issues. For example, effective capacity planning in imaging services should enable improved patient access balanced against the need to avoid over-use of interventions that have the potential to cause harm, such as ionising radiation.