The Compact Linear Collider (CLIC) is a concept for a future linear particle accelerator that aims to explore the next energy frontier. CLIC would collide electrons with positrons and is currently the only mature option for a multi-TeV linear collider. The accelerator would be between long, more than ten times longer than the existing Stanford Linear Accelerator (SLAC) in California, USA. CLIC is proposed to be built at CERN, across the border between France and Switzerland near Geneva, with first beams starting by the time the Large Hadron Collider (LHC) has finished operations around 2035. The CLIC accelerator would use a novel two-beam acceleration technique at an acceleration gradient of 100 MV/m, and its staged construction would provide collisions at three centre-of-mass energies up to 3 TeV for optimal physics reach. Research and development (R&D) are being carried out to achieve the high precision physics goals under challenging beam and background conditions. CLIC aims to discover new physics beyond the Standard Model of particle physics, through precision measurements of Standard Model properties as well as direct detection of new particles. The collider would offer high sensitivity to electroweak states, exceeding the predicted precision of the full LHC programme. The current CLIC design includes the possibility for electron beam polarisation. The CLIC collaboration produced a Conceptual Design Report (CDR) in 2012, complemented by an updated energy staging scenario in 2016. Additional detailed studies of the physics case for CLIC, an advanced design of the accelerator complex and the detector, as well as numerous R&D results are summarised in a recent series of CERN Yellow Reports. Background There are two main types of particle colliders, which differ in the types of particles they collide: lepton colliders and hadron colliders. Each type of collider can produce different final states of particles and can study different physics phenomena. Examples of hadron colliders are the ISR, the SPS and the LHC at CERN, and the Tevatron in the US. Examples of lepton colliders are the SuperKEKB in Japan, the BEPC II in China, DAFNE in Italy, the VEPP in Russia, SLAC in the US, and the Large Electron–Positron Collider at CERN. Some of these lepton colliders are still running. Hadrons are compound objects, which lead to more complicated collision events and limit the achievable precision of physics measurements. This is for instance why the Large Hadron Collider was designed to operate at such a high