This presentation will describe how a fiber-optic turbidity probe may be used as an inferential sensor for feedback control of commercial-scale batch crystallizations. The discussion will focus on several unseeded crystallization examples involving cooling or cooling plus addition of antisolvent. In a typical control strategy, the probe is used to detect an initial nucleation event, to track a subsequent digestion step for fines dissolution with potential for modification of nuclei size and purity, and then to monitor a growth period. During these steps, operating variables that affect generation of supersaturation are adjusted to maintain a desired turbidity versus time profile. The profile can be determined in liter-scale experiments and then replicated at the 10,000+ liter commercial scale. Within Dow, this control strategy has proven to be robust and cost-effective for numerous commercial-scale batch crystallizations implemented during the past 20 years, including those with highly fouling or corrosive environments.