Work over the past four decades clearly indicates that the Metzner-Otto approach has been the single major practical technique for incorporating non-Newtonian effects in a variety of processes. It has mainly been used in the field of mixing for the correlation of results for non-Newtonian fluids relating to power requirement, heat transfer, and blend time. As anticipated, it has been most successful in situations where the process of interest is controlled by the fluid motion in the vicinity of the impeller. A major application has been in the development of unique rheometers for systems which are not readily handled by conventional devices. The role of viscoelasticity has not been definitively established and is an area for future work.