Flow visualisation is used for many applications to supplement measured forces and pressures as a means of understanding particular features of the flow. This Data Item is primarily concerned with oil flow visualisation applied to wind tunnel testing of aircraft configurations. There is also some discussion of the sublimation technique, used to identify boundary layer transition. The use of oil flow visualisation as a means of identifying flow features can be a valuable tool to aid the analysis of the aerodynamic characteristics of a configuration. The procedure for carrying out flow visualisation testing has been developed in many test facilities and is described here only to the extent necessary to identify features of particular techniques which enhance the ability to interpret the visualisation and to highlight problems which can limit the value of the results as an aid to aerodynamic analysis. A description is given of the choice of the technique, insofar as it affects the requirements of the anticipated aerodynamic analysis, together with some warnings of those factors that may limit the value of oil flow tests. A separate section shows details of particular features that may be encountered in the analysis of oil flow results and defines the terms used to describe the features. However, the greater part of the Item presents and discusses examples of flow patterns. A number of civil and military aircraft configurations have been chosen to demonstrate many of the characteristics which can be identified from flow visualisation investigations. In general, the results are presented and discussed in the order of increasing configuration and flow complexity.