Chemists and engineers involved in the prooduction of solid drugs have to deal with difficult new challenges, including the on-line mastery of crystal habits and size distributions or the control of polymorphic transitions. A major limitation to improving the control of industrial crystallization of drugs lies in the lack of versatile and reliable on-line sensors. Supersaturation measurements can be performed using in situ infrared spectroscopy. New applications of the technique are described : the acquisition of key-data characterizing the solute-solvent system in question, the design of improved monitoring or operating strategies and the monitoring of polymorphic transformations during crystallization operations.