An electron-positron linear collider operating in the centre of mass range 500 GeV to O(1 TeV) will provide high precision measurements of particle properties and production cross-sections at TeV energies.  These measurements, together with those at the LHC, will produce a complete picture of physics at the TeV scale.  The currently proposed machines are described and a few key physics measurements are presented  (with special emphasis on Higgs physics and supersymmetry.)