This Technical Specification gives the standard test method for determining residual stresses in polycrystalline materials by neutron diffraction. It is applicable to homogeneous and inhomogeneous materials and to test pieces containing distinct phases. The principles of the neutron diffraction technique are outlined. Advice is provided on the diffracting lattice planes on which measurements should be made for different categories of materials. Guidance is provided about the directions in which the measurements should be obtained and of the volume of material, which should be examined, in relation to material grain size and the stress state envisaged, when making measurements. Procedures are described for accurately positioning and aligning test pieces in a neutron beam and for precisely defining the volume of material that is sampled when individual measurements are being made. The precautions needed for calibrating neutron diffraction instruments are described. Techniques for obtaining a stress free reference are presented. The methods of making individual elastic strain measurements by neutron diffraction are described in detail. Procedures for analysing the results and for determining their statistical relevance are presented. Advice is provided on how to determine reliable estimates of residual (or applied) stress from the strain data and of how to estimate the uncertainty in the results.