Simple experimental tests of fatigue life are often insufficient to characterise fatigue behaviour. Fatigue crack growth in polycrystalline metals is governed by a number of interacting mechanical effects at the crack tip, such as the deformation inside a plastic zone and contact between the crack faces over part of the loading cycle. Typically, results tend to be interpreted in terms of an empirical fatigue law such as the Paris equation, which in itself fails to generalise to different load ratios or multiaxial load cases. While extensions to this equation have been used, these are mostly empirical and do little to enhance understanding of the fatigue growth mechanisms. Recently, the use of diffraction to characterise crack tip stress effects has become increasingly popular. In this paper, we consider the opportunities and the difficulties associated with making such measurements by neutron and synchrotron diffraction. In particular we examine grain size effects, plane stress/plane strain issues, optimisation of the gauge geometry, measurement of the plastic zone and crack closure effects.