The　rising　demand　for　radiation　detection　materials　in　many　applications　has　led　to　extensive　research　on　scintillators(1-3).　The　ability　of　a　scintillator　to　absorb　high-energy　(kiloelectronvolt-scale)　X-ray　photons　and　convert　the　absorbed　energy　into　low-energy　visible　photons　is　critical　for　applications　in　radiation　exposure　monitoring,　security　inspection,　X-ray　astronomy　and　medical　radiography(4,5).　However,　conventional　scintillators　are　generally　synthesized　by　crystallization　at　a　high　temperature　and　their　radioluminescence　is　difficult　to　tune　across　the　visible　spectrum.　Here　we　describe　experimental　investigations　of　a　series　of　all-inorganic　perovskite　nanocrystals　comprising　caesium　and　lead　atoms　and　their　response　to　X-ray　irradiation.　These　nanocrystal　scintillators　exhibit　strong　X-ray　absorption　and　intense　radioluminescence　at　visible　wavelengths.　Unlike　bulk　inorganic　scintillators,　these　perovskite　nanomaterials　are　solution-processable　at　a　relatively　low　temperature　and　can　generate　X-ray-induced　emissions　that　are　easily　tunable　across　the　visible　spectrum　by　tailoring　the　anionic　component　of　colloidal　precursors　during　their　synthesis.　These　features　allow　the　fabrication　of　flexible　and　highly　sensitive　X-ray　detectors　with　a　detection　limit　of　13　nanograys　per　second,　which　is　about　400　times　lower　than　typical　medical　imaging　doses.　We　show　that　these　colour-tunable　perovskite　nanocrystal　scintillators　can　provide　a　convenient　visualization　tool　for　X-ray　radiography,　as　the　associated　image　can　be　directly　recorded　by　standard　digital　cameras.　We　also　demonstrate　their　direct　integration　with　commercial　flat-panel　imagers　and　their　utility　in　examining　electronic　circuit　boards　under　low-dose　X-ray　illumination.