Variability inherent in the electrophotographic (EP) process often mandates some kind of process control strategy to provide the required image consistency. Both photoconductor and toner generally exhibit substantial unit-to-unit variability and environmental sensitivity, plus variability over the rest-run duty cycle and long-term aging. Another challenge addressed by process control is maximizing toner yield (prints from a given quantity of toner) without compromising image quality. Many process control strategies rely on on-board densitometers to monitor the process and provide the basis for automatic process control adjustments. Laboratory bench-top densitometers are too big and expensive, and have more capability than needed for process control. The specialized on-board densitometer typically doesn't need the user interface, nor certain other features, and the cost of the on-board densitometer must be reasonable vis-a-vis the total print engine cost. However, in certain respects the on-board densitometer may have to meet higher performance standards than the laboratory instrument. This paper provides an overview of on-board densitometer performance requirements and technology trends, in dry electrophotographic applications.