ESDU 90029 provides information to assist in the application of the group of ESDU data on the prediction of the drag due to such excrescences as grooves, ridges and steps, rivets, and cylinders and stub wings immersed in the boundary layer. Those data were obtained from wind-tunnel tests of an idealised condition in which the excrescence was mounted on a flat plate with no pressure gradient, and the application of such data to the real aircraft situation is considered. Corrections are required for the effect of pressure gradient and flow orientation. The pressure gradient correction on a lifting surface is a magnification factor (although it can be less than unity) and its estimation in incompressible two-dimensional flow is discussed; its variation with the boundary-layer parameters on an aerofoil is illustrated graphically. The analysis is extended to compressible flow and reference is made to the prediction methods of ESDU 87004, 91028 and 91029. The treatment of an excrescence only partially immersed in the boundary layer is also considered, and recommendations are made for the case of excrescences on non-lifting components. The effect of flow orientation is more briefly discussed, and suggestions are made for some situations, while the treatment of an excrescence for which no specific data are available is also considered. The significance of the excrescence drag penalty on aircraft performance is discussed, and acceptable levels in current good design practice are suggested. Reference is made to ESDU 93032 in which the excrescence drag of the wing of a subsonic transport aircraft at the cruise condition is considered as a detailed set of examples in which the data from a number of Data Items are utilised.