A pair of basic methods for investigating the theological properties of complex fluids were considered: steady flow and small amplitude oscillatory shear. The methods were applied to study rheological properties of eight petroleum samples belonging to different classes (light, heavy, waxy crude oils, and bitumen). It was shown how the-viscosity of a crude oil depends on its nature, temperature, and applied stress. To characterize the viscous behavior of oils, one can use their glass transition temperature: the Williams-Landel-Ferry equation makes it possible to construct a universal temperature dependence of the viscosity. The latter determines the viscosity of any petroleum at temperature of interest from that at some other temperature. Linear and nonlinear viscoelastic properties of oils were demonstrated, and the manifestations of viscoelasticity by various crude oils were shown. It was revealed how the pour point of heavy oil can be calculated from its viscosity.