Stress/strain determination by x-ray or neutron diffraction means always to measure lattice plane distances in different direction and then to solve a system of simultaneous., linear equations. The number of unknowns and equations needed is up to six, but can be smaller if there is already some information about the stress/strain state, e.g. if principal axes are known or if there is a stress free surface. No matter how many equations one has to solve, the accuracies of the calculated unknowns depend not only on the accuracies of the measured lattice plane distance but also very much on the geometrical relationships among the measurement directions. Therefore the whole measurement must be thoroughly planned, it is that the measurement directions must be choosen thoroughly. In the case of a textured or a single crystalline material this is quite difficult and should be done using computer programs. Such programs are intruduced together with some examples.