We review previous X-ray diffraction experiments on porous silicon, giving a determination of porous layer strain. The results of our X-ray measurements of the strain of as-formed and anodically oxidized samples are presented; the diffraction curves of thin layers and of superlattices and the strain induced by liquid wetting and vapour adsorption have been measured. The size and shape of silicon crystallites can be determined from the X-ray diffraction pattern, in particular by using reciprocal space maps.