X-ray and neutron diffraction are particularly useful for characterizing ferroelectric materials in situ, e.g., during application of temperature, pressure, electric field, and stress. In this review, we introduce many experimental approaches for such measurements and highlight important discoveries in ferroelectrics that utilized diffraction. We focus our examples on polycrystalline ferroelectrics, though many of the approaches and analysis methods can also be applied to thin films and single crystals. Methods discussed for characterization of structure include, phase identification, line profile analysis, whole pattern fitting, pair distribution functions, and the x-ray diffraction based three-dimensional microscopy. Further advancement of these and other techniques offers potential for continued important contributions to the fundamental understanding of ferroelectric materials.