ESDU 97034 is one of the group of ESDU Data Items on the flow Characteristics of non-Newtonian fluids, and provides an introductory guide to the identification, classification and characterisation of non-Newtonian fluids, including measurement methods. Fluids are classified firstly by their phase condition (as homogenous, pseudohomogenous or heterogeneous) and secondly on the basis of their behaviour when subjected to shear. Guidance is then given on the main classes of flow behaviour exhibited over limited ranges of shear rate under steady-state shear: these classes are Newtonian, shear-thinning, shear-thickening, Bingham plastic and viscoplastic, Generally, most fluids exhibit behaviour conforming to more than one flow classification over a shear rate range wider than two or three decades, and this behaviour is discussed and illustrated by typical flow curves. Flow curve models are presented that are used to characterise the idealised forms of flow behaviour (over limited shear rate ranges) and the behaviour of real fluids over wider shear rate ranges. A brief discussion is given on commercial viscometers and rheometers, employed for viscosity and flow curve measurement, and includes guidance on practical aspects of their use when applied to typical fluid types. A glossary of viscometric terms is provided in an Appendix. Companion Data Item ESDU 95012 provides guidance on the selection and use of the different viscometric geometries available and advice on obtaining the viscometric data necessary for the generation of the flow curve. Methods for generating the flow curve from raw experimental data using a tube viscometer are illustrated by the worked example of ESDU 04005. The flow curve is required by Data Item ESDU 91025 which provides methods and computer programs for the estimation, using the flow curve and by scale-up of small-scale pipeline flow data, of frictional pressure drop in fully-developed, isothermal flow in straight pipes of uniform circular cross section.