This paper reviews the need for fast engine warm-up, and quantifies the fuel consumption penalty from cold engine operation, through an examination of vehicle usage and fuel consumption during warm-up. The conclusion is, that this is one of the few areas where there is still a potential (perhaps as high as 15%) for a reduction in overall fuel consumption. The reasons for poor engine performance during warm-up are discussed, and it is argued that the poor mixture preparation is the dominant effect.An experimental system is described, here, that has produced measurements of the heat fluxes from the combustion chamber, the heat transfer coefficient to the coolant, and key temperatures, for a range of operating points during warm-up. The heat flux and heat transfer coefficients are slightly dependent of the coolant temperature, but the observed trends are explained by reference to a correlation for convective heat transfer. It has also been shown, that the metal temperatures around the combustion chamber rise very rapidly to close to their equilibrium values. This supports the supposition that mixture preparation is the main cause of high fuel consumption during warm-up. The excess fuel consumption is partly due to the need to establish the fuel film on the inlet manifold wall, and a comparison has been made between the instantaneous fuel flowrate into the carburettor with that being burnt.