The Metzner-Otto concept has been applied for nearly 50 years in almost every process engineering publication dealing with power characteristics of stirrers in non-Newtonian fluids. Although in some cases the usefulness of this concept has been questioned, a fundamental discussion about this method has so far not proved necessary. However, the principal question whether such a correlation is well-founded from the standpoint of dimensional analysis has not yet been discussed. In this paper, the dimensional analytical background of the MO concept is critically studied and the method illustrated by way of examples.