Abstract1.The plastic behaviour of suspensions is considered from the point of view of energy, of structure and of mechanics. According to this method of consideration the systems which exhibit so‐called Ostwald structural viscosity (Ostwaldsystems) are to be conceived of as suspensions with elasticallydeformablemicelles, while the so‐called Binghamsystemsare to be conceived of as suspensions of complexes which exhibitdistintegrationduring stationary flow.2.The similarity and the difference is pointed out between our method of analysis of theD‐τ data of suspensions of the Bingham type and the method of analysis proposed by Williamson.3.The fluidity (reciprocal of the viscosity) is determined for Ostwald systems by\documentclass{article}\pagestyle{empty}\begin{document}$ \frac{{\dot D}}{\tau } $\end{document}, for Bingham systems by\documentclass{article}\pagestyle{empty}\begin{document}$ \frac{{d\dot D}}{{d\tau }} $\end{document}.4.The fluidity‐shearing stress (φ‐τ) diagram, related in a simple way to theD‐τ data, enables us to establish theplastic behaviour of suspensionsin general with the help of the same constants (liquidation tendency L and plasticity