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This article was printed in Hydrocarbon Engineering magazine dated April 1999 and describes the flow of non-Newtonian fluids in heat exchangers. For many years, Hook's law for solids and Newton's law for liquids were considered universal. In classical mechanics, material properties were considered known when either viscosity or modulus of elasticity were given; the basic 'constitutive equations' governing the behavior of each small material element were known. Practical analysis often involved coupling these equations with the momentum equation and applying them to the increasingly complicated flows or deformations. However, dealing with solids and liquids seemed deceptively simple. As nature is never so straightforward, this was a gross oversimplification of a complex situation. In virtually all industries including petroleum, foods, pharmaceuticals, minerals, etc, many materials cannot be strictly classified as liquids or solids. The science that deals with these complex materials is called 'Theology', the study of the deformation and flow of matter.