An exhaust system(10) for a lean-burn internal combustion engine (12) comprises a first substrate monolith (16) comprising a catalyst for oxidising nitric oxide (NO) comprising a catalytic oxidation component followed downstream by a second substrate monolith (18) which is a wall-flow filter having inlet channels and outlet channels, wherein the inlet channels comprise a NO x absorber catalyst (20) and the outlet channels comprise a catalyst for selective catalytic reduction (22) of nitrogen oxides with nitrogenous reductant.