This thesis is concerned mainly vTith the different regimes of rheological behaviour exhibited by elastohydrodynamic h ;lbricants, and the measurement of its constituent properties as functions of the contact conditions. From reviewing all the known applicable rheometrical techniques it was evident a line contact disc machine employing heated discs offered the greatest potential for investigation work. Using this apparatus a method was developed for directly producing isothermal traction curves. When plotted in terms of shear stress versus log strain rate their shape provided a clear indication of the type of rheological behaviour. These gave support to the Johnson and Tevaarwerk viscoelastic constitutive equation, also at high pressures a limiting value of shear. stress was reached and the fluid sheared plastically. These isothermal and isobaric traction curves were fitted to the Johnson and Tevaarwerk model system by using the appropriate averaged rheological properties of: 'limiting Newtonian shear stress', 'dynamic viscosity' and 'limiting shear stress' as disposable parameters, thereby establishing their values as functions of pressure and temperature. Rheological behaviour and properties were found to be almost independent of rolling speed. In order to provide a means of identifying the regime of rheological behaviour from the operating conditions, without the need to perform specific traction tests, the concept of a regime chart was devised. These were constructed by plotting the physical limitations of each type of behaviour on axes containing all the relevant EHL parameters; specific examples were made for all three test fluids. In developing this a coincidence between the conditions for the onset of plastic shearing in EHL and the glass transition state of the fluids became apparent. Finally, the influence of rough surfaces on traction was analysed using the rheological work as a basis. This was done in terms of the parameter A, the ratio of theoretical film thickness for perfectly smooth surfaces to the combined asperity heights, i.e. (h/o). Three distinct forms of behaviour were observed: (a) for A > la 'Film film' EHL traction where \l < \lc (b) for 1 < A < la 'Asperity' EHL tracti on where \l \lc (c) for A < 1 Boundary lubrication traction where \l > \l c A simple theoretical treatments of the surface profile yielded an approximate average asperity pressure, and for strictly asperity EHL conditions, smooth surface traction data at equivalent pressures confirmed this to be reasonably accurate. ii