Two examples demonstrate how X-ray diffraction can be used to characterize internal particle parameters including crystal structure, crystallite size and microstrain. After cocrystallization of pharmaceutical ingredients by ball milling and supercritical processes new phases and strained crystal lattices were found with crystallite sizes in the submicro to nanometer range. For two energetic materials the crystal to microstructure relation and the influence of comminution techniques on domain size and microstrain were investigated. Both examples focused on advanced particle engineering by understanding the function and design of crystalline constituents.