Finding the characterization method that best matches the pulping process is crucial in enabling the monitoring and control of pulp and final-product properties. Control of optimal process parameters in the production line are thus dependent on reliable and valid results. The Crill method is an analogue method based on the relationship between the interactions of lights of two different wavelengths (UV and IR) with the particles in the pulp. By comparing the specific UV and IR surfaces, the crill value is calculated. The crill method has been used in two studies with the aims to validate the crill measurement in a high yield pulping (HYP) process by focusing on refining and control of crill value of different pulp streams. The results indicate that the crill method can be used to monitor fibre treatment in refining processes. Moreover, by controlling pulp streams with regards to crill value, improvements of pulp strength and retention on the paper machine (PM) can be achieved.