We review recent progress in the study of the interfacial properties of Si/Si_1-xGe_x heterostructures by x-ray scattering techniques. The evolution of interfaces in Si/Si_1-xGe_x superlattices as a function of growth temperature in the range 250-750 °C is discussed. It is shown that at 250^oC the interfaces are chemically abrupt, but are highly defected and exhibit a pronounced vertically correlated physical roughness. Growth at intermediate temperatures (400-550 °C) results in structures with good crystallinity and physically smooth interfaces, but intermixed over at least two monolayers. Intermixing is also more pronounced at the Si_1-xGe_x to Si interfaces. Higher growth temperatures (620-750 °C) further enhances the intermixing and causes undulation (100 nm length scale) of the interfaces. The interface perfection in thin (Si_mGe_n)_p atomic layer superlattices is also examined. In these structures the interfaces have a width of about two monolayers and exhibit vertical correlation related to the substrate surface morphology. Interdiffusion in different types of Si/Ge heterostructures is also investigated. The measurements show that Ge diffusion exhibit a strong anisotropy. Upon annealing, diffusion causes a gradual broadening and a decrease of the Ge content of the Ge-rich regions, while sharp concentration gradients are preserved at the Si/alloy interfaces. The thickness fluctuations in these Si/Ge multilayers are also studied and are estimated about 1-2 % in thick Si/Si_1-xGe_x superlattices and about 5% in (Si_mGe_n)_p atomic layer superlattices. Finally, Ge surface segregation, involving about 2 monolayers is found in Si/Ge heterostructures.