As regards conditions in the mould, Continuous Casting of 400 mm thick slabs can differ from conventional slab casting. Specific features of thick slab casting are reported, concerning heat fluxes, both integral and local. Local heat flux has been determined through an instrumented mould along with thermal modeling of the copper plate. Integral heat withdrawal data are compared with data from conventional slab casting. The effect of steel composition on integral and local heat extraction has been evaluated and interpreted in relation to residence times in the mould and to the relevant thermal resistances. The influence of tundish temperature on superheat in the mould and on heat removal was quantified. Oscillation mark depth and mould powder consumption have been related to heat withdrawal. Cracking defects were characterized in dependence on mould heat flux densities.