Diffraction intensities can be evaluated by two distinct procedures: summation integration and profile fitting. Equations are derived for evaluating the intensities and their standard errors for both cases, based on Poisson statistics. These equations highlight the importance of the contribution of the X-ray background to the standard error and give an estimate of the improvement which can be achieved by profile fitting. Profile fitting offers additional advantages in allowing estimation of saturated reflections and in dealing with incompletely resolved diffraction spots.