ESDU 97020 gives a method for predicting the linear viscous lift-curve slope of symmetric and cambered aerofoils in subcritical compressible flow. The VGK program (see ESDU 96028 and 96029 in the Transonic Aerodynamics Series) was applied to a range of NACA aerofoils and also to aerofoils with complex camber lines to derive graphs of a reduction factor to be applied to the inviscid lift-curve slope from ESDU 72024. Investigation of the effect of aerofoil geometry on lift-curve slope showed that geometry effects on the reduction factor could be adequately characterised by the use of an equivalent trailing-edge angle based on thickness at 0.9 and 0.99 of chord for the symmetric aerofoils, with the additional requirement of a trailing-edge upper surface angle based on ordinates at the same positions for the cambered aerofoils. The method also requires the assumption of transition positions on the upper and lower surfaces, and the Reynolds and Mach numbers. It applies for fully-attached boundary layers but becomes unreliable if lift-curve slope is strongly dependent on transition movement with angle of attack. It predicts the reduction factor to within 0.005. A fully worked example illustrates the use of the method and the aerofoils used in its derivation are listed.