The Ultrafast X-ray Summer School (UXSS) 2023 will take place in-person at DESY in Hamburg, Germany, 12-16 June 2023. UXSS is an annual event jointly organized by the Center for Free-Electron Laser Science (CFEL) at DESY, Hamburg, Germany, the Stanford PULSE institute at SLAC National Accelerator Laboratory, Menlo Park, California, USA and Paul Scherrer Institute (PSI), Switzerland.