A microindenter in the form of a flat cylindrical punch (10 mum < diameter < 100 mum) is used to measure the interfacial shear strength of polymer coatings on relatively rigid substrates. The penetration of the displacement controlled tip in the polymer results in a plastic zone ahead of the moving tip, which generates a large interfacial shear stress. An analysis based on a modified shear lag model can be used to estimate the critical shear stress at which failure of the interface initiates. The test can be used for hard and soft polymers such as polymethylmethacrylate, polystyrene and high impact polystyrene. The thickness of coating which can be tested is shown to be approximately equal to the diameter of the tip.