Ideally, the active pharmaceutical ingredient has a single well-characterized and relatively stable crystal structure. However, complications can arise for molecules that crystallize in several solvated and/or neat forms which may de-solvate/transform and/or interconvert during manufacturing processes. Single crystal studies of these forms define their structure/composition and moreover allow PXRD simulations and correlation of spectra with structure (Raman, IR) which can be used in the determination of thermodynamic relationships, the mapping of design space, and ultimately to improve process control. Some recent examples will be presented, with emphasis on process relevant forms of dasatinib.