An exhaust system for an internal combustion engine (28) for controlling the release of undesirable emissions from the engine comprises an exhaust pipe (32) for receiving an exhaust flow from the engine, an SCR catalyst (48) arranged in the exhaust flow and means (80) for determining the temperature of the SCR catalyst. A NOx absorber (38), such as a diesel oxidation and NOx absorber catalyst (DONAC), is located in the exhaust flow at a position upstream of the SCR catalyst (48) for absorbing and releasing NOx contained in the exhaust flow. Means is provided for controlling the NOx absorber (38) so as to control the release of NOx to the SCR catalyst (48) in dependence on the temperature of the SCR catalyst, thereby to effect active management of release of NOx from the DONAC (38).