ESDU 92039 discusses the qualitative effects of control gaps on wing aerodynamic characteristics, explains the sources of drag, and considers the benefits of sealing. The effects of wing section and control section profile mismatch on control gap drag are also considered. The limited data in the literature are used to develop empirical prediction methods for the drag due to unsealed spanwise gaps while for unsealed chordwise gaps, for which the data are too few to be correlated, guidelines on upper limits to drag are given. Methods of allowing for the effect of sealing are suggested in both cases. The data apply to controls or flaps with ratios of control or flap chord to wing chord of 0.1 to 0.3 at the control or flap mid-span and hinge lines swept by up to 30 degrees. The methods apply to wing lift coefficients between 0 and 0.6 and are independent of Mach number (provided the flow is everywhere subsonic) and of Reynolds number for gap depth-to-width ratios equal to or exceeding 3. The accuracy of the methods is assessed, and sketches compare experimental values with prediction. A worked example illustrates the use of the methods and highlights the need for gap sealing.