ESDU AERO W.02.04.00 introduces a number of ESDU documents dealing with the drag of aerofoils. ESDU AERO W.02.04.01 gives the drag of a flat plate for use with ESDU AERO W.02.04.02 and ESDU AERO W.02.04.03 which provide a method of estimating profile drag by factoring that flat plate drag by a form factor. The limitations of such an approach are discussed and the importance of transition position in determining the drag is emphasised. Various influences on transition position are considered and, in particular, ESDU 73016 and ESDU AERO W.02.04.11 which give maximum sizes of waviness and of roughness that have no effect on transition position are introduced. The concept of equivalent sand roughness is discussed. Attention is also drawn to ESDU 73016 which gives the maximum size of roughness that gives no increase in skin friction drag and allows the increase in skin friction drag to be calculated if that limit is exceeded. Also mentioned are a group of ESDU documents that treat the increase in drag due to a wide range of differently-shaped excrescences.