A diffuser for diffusing the exhaust gas produced by an engine, said diffuser comprising a wall or walls defining sections of the diffuser, said diffuser comprising: an initial section which extends for substantially one third the length of the diffuser, at the entry to said initial section the shape factor of the boundary layer flow at the or each wall of the section being in the range 1.3 to 1.6, said initial section being configured so as to substantially linearly increase said shape factor so that at the exit of said initial section said shape factor is in the range 1.6 to 2.2; and a main section following the initial section and extending for the remaining length of the diffuser, said main section being configured so as to maintain said shape factor in the range 1.6 to 2.2 over the length of the main section.