A Selective Catalytic Reduction system comprising a system inlet, a central tube, a gas flow system and a plurality of catalyst clusters; wherein: a. the system inlet comprises an inlet, which widens laterally to an initial mixing zone, the inlet being configured to receive exhaust gases from an engine and being in fluid communication with the mixing zone, the mixing zone being defined in part by a cover and an inlet cone narrowing to, and being in fluid communication with, a central tube, the mixing zone comprising a plurality of means for introducing a reductant, or a reductant precursor, into exhaust gas in the mixing zone, the mixing zone comprising at least one surface in thermal communication with the gas flow system; b. the central tube being in fluid communication with the system inlet and the gas flow system; c. the gas flow system comprises (i) a gas flow deflector, (ii) a plurality of passages, and (iii) an outlet zone comprising an outlet cone and an outlet in fluid communication with the outlet cone, the gas flow deflector being in fluid communication with the central tube and either: i. a plurality of catalyst clusters, wherein the plurality of catalyst clusters is in fluid communication with a plurality of passages in fluid communication with the outlet zone; or, ii. a plurality of passages in fluid communication with a plurality of catalyst clusters, wherein the plurality of catalyst clusters are in fluid communication with the outlet zone; and d. each catalyst cluster comprises a SCR catalyst. a. the system inlet comprises an inlet, which widens laterally to an initial mixing zone, the inlet being configured to receive exhaust gases from an engine and being in fluid communication with the mixing zone, the mixing zone being defined in part by a cover and an inlet cone narrowing to, and being in fluid communication with, a central tube, the mixing zone comprising a plurality of means for introducing a reductant, or a reductant precursor, into exhaust gas in the mixing zone, the mixing zone comprising at least one surface in thermal communication with the gas flow system; b. the central tube being in fluid communication with the system inlet and the gas flow system; c. the gas flow system comprises (i) a gas flow deflector, (ii) a plurality of passages, and (iii) an outlet zone comprising an outlet cone and an outlet in fluid communication with the outlet cone, the gas flow deflector being in fluid communication with the central tube and either: i. a plurality of catalyst clusters, wherein the plurality of catalyst clusters is in fluid communication with a plurality of passages in fluid communication with the outlet zone; or, ii. a plurality of passages in fluid communication with a plurality of catalyst clusters, wherein the plurality of catalyst clusters are in fluid communication with the outlet zone; and i. a plurality of catalyst clusters, wherein the plurality of catalyst clusters is in fluid communication with a plurality of passages in fluid communication with the outlet zone; or, ii. a plurality of passages in fluid communication with a plurality of catalyst clusters, wherein the plurality of catalyst clusters are in fluid communication with the outlet zone; and d. each catalyst cluster comprises a SCR catalyst.