A nacelle for a turbofan gas turbine engine, having in flow series an intake lip, a diffuser and a fan casing. The diffuser has, in flow series, a main section and a recessed section adjacent to the fan casing. A transition region, such as a step, slope or curve, is provided between the main section and the recessed section. In some arrangements the transition region may be configured to promote flow separation and the formation of a separation bubble in the recessed section.