Self-affine fractals offer a tool for characterising surface roughness. Fractal parameters are scale-invariant and can be derived from the Structure Function (SF): if computation of the SF is not practicable then parameters can be derived from roughness parameters available on many commercial software packages. The three-dimensional SF gives information about anisotropy. SFs can offer a demonstration of the effect of wear on surface topography in tribological investigations. Finally the SF can be used with material properties to obtain a low-pass cutoff for the calculation of surface data for use in rough contact mechanics.