CompactLight is a Design Study funded by the European Union under theHorizon 2020 research and innovation funding programme, with Grant Agreement No. 777431.CompactLight was conducted by an International Collaboration of 23 internationallaboratories and academic institutions, three private companies, and five third parties.The project, which started in January 2018 with a duration of 48 months, aimed to designan innovative, compact, and cost-effective hard X-ray FEL facility complemented by asoft X-ray source to pave the road for future compact accelerator-based facilities. Theresult is an accelerator that can be operated at up to 1 kHz pulse repetition rate,beyond today's state of the art, using the latest concepts for high brightness electronphotoinjectors, very high gradient accelerating structures in X-band, and novelshort-period undulators. In this report, we summarize the main deliverable of theproject: the CompactLight Conceptual Design Report, which overviews the current statusof the design and addresses the main technological challenges.