As regards conditions in the mould, Continuous Casting of 400mm thick slabs can differ from conventional slab casting. Specificfeatures of thick slab casting are reported, concerning heat fluxes,both integral and local. Local heat flux has been determinedthrough an instrumented mould along with thermal modeling ofthe copper plate. Integral heat withdrawal data are compared withdata from conventional slab casting. The effect of steelcomposition on integral and local heat extraction has beenevaluated and interpreted in relation to residence times in themould and to the relevant thermal resistances. The influence oftundish temperature on superheat in the mould and on heatremoval was quantified. Oscillation mark depth and mould powderconsumption have been related to heat withdrawal. Crackingdefects were characterized in dependence on mould heat fluxdensities.