An exhaust system for an internal combustion engine, the exhaust system comprising, a lean NOx trap, a NOx storage and reduction zone on a wall flow monolithic substrate having a pre-coated porosity of 50% or greater, the NOx storage and reduction zone comprising a platinum group metal loaded on one or more first support, the or each first support comprising one or more alkaline earth metal compound, and a selective catalytic reduction zone on a monolithic substrate, the selective catalytic reduction zone comprising copper or iron loaded on a second support, the second support comprising a molecular sieve.