The Large Hadron Collider (LHC) is one of the most powerful scientific instruments ever built. It has been exploring the energy frontier since 2010, brought together a global user community of more than10’000 scientists and led to the experimental discovery of the Higgs boson in 2012. The LHC will reach its lifetime end in 2025 when it will have accumulated an integrated luminosity of approximately 450fb-1 (total data set delivered to the experiments). To extend its physics reach, in particular with regard to the Higgs boson and the structure and evolution of the universe, the LHC requires a major upgrade to increase its instantaneous luminosity (rate of proton-proton collisions) by a factor of 5 beyond its nominal design value, and the integrated luminosity by a factor of 10. This is the goal of the High-Luminosity LHC (HL-LHC), which will extend the LHC operation until 2041. Being a highly complex and technologically challenging machine, such an upgrade required about 10 years of R&D before it could be implemented. HL-LHC relies on a number of key innovative technologies, each representing exceptional technological challenges, such as: cutting edge 11-12 Tesla superconducting Nb3Sn quadruple magnets, very compact superconducting RF (radio-frequency) cavities for beam rotation at the interaction points with ultra-precise phase control, new technology for beam collimation, high current superconducting links with negligible energy dissipation, very precise high-current power converters with energy recovery, new surface treatment of the beam screen for electron-cloud suppression, remote controlled robotics and intervention tools. The HL-LHC project started in 2011 as an EU funded Design Study that identified the key upgrade strategies, and was formally approved by the CERN Council in 2016. It required the construction of new access shafts and underground galleries (for a total of 1.5 km) and caverns to house the new equipment and to facilitate access and maintenance activities during machine operation, and new surface buildings to house new equipment like cryogenic refrigeration and industrial cooling and ventilation units. The civil engineering work started in 2018 and it was completed in 2022 on time and budget. The HL-LHC components are scheduled to be installed during the third Long Shutdown period of the LHC, starting in 2026. The start of the HL-LHC operation period is planned for 2029. The project has an overall material cost of about 1,600 MCHF and requires about 2,200 FTEs over a period of ca. 12 years. From the beginning, the HL-LHC has been devised as an international project, with about 10% of its budget coming from external institutes and international collaborations via in-kind contributions. The HL-LHC project is complemented by an upgrade of the LHC injector complex, which was successfully implemented during the second LHC Long Shutdown from 2018 to 2021, and by upgrade projects of the LHC detectors, carried out in phases.