Also displayed in Fig.~\ref{fig:lumi} are the luminosities expected for the two linear collider projects, ILC~\cite{ILC:AcceleratorA,ILC:AcceleratorB} and CLIC~\cite{cite:CLICDR}, as a function of the centre-of-mass energy. It is remarkable that the luminosity expected at TLEP is between a factor 5 and three orders of magnitude larger than that expected for a linear collider, at all centre-of-mass energies from the Z pole to the $\ttbar$ threshold, where precision measurements are to be made, hence where the accumulated statistics will be a key feature. Upgrades aimed at delivering luminosities well beyond the values given above are also being investigated -- although they cannot be guaranteed today. Possibilities include beam charge compensation and the use of the “crab-waist” collision scheme~\cite{TelnovC, TelnovD}, allowing beamstrahlung effects to be mitigated. Upgrades to higher centre-of-mass energies are discussed in Section~\ref{sec:VHE-LHC}. 
