Because of very important reserves that are of the same order of magnitude than conventional oils, heavy oils represent a strategic source of hydrocarbons. The major difficulty for producing and transporting these crudes comes from their very high viscosities. The success of exploitation of these petroleum products requires new treatments to improve their transport. Such a target implies to better understand the relationship between the composition of heavy oils, in particular in terms of asphaltenes and resins content and their flow properties. Influence of asphaltenes content in the crude has been particularly studied in our lab : the experimental work revealed the existence of a critical concentration C* above which the asphaltenes particles overlapp. This structural change dramatically increases the viscosity and intensifies the elastic character of heavy crude oils. A further series of experiments has focused on the influence of resins: they show that in dilute regime (C<C*), resins increase the viscosity of the crude and in concentrated regime (C>C*), which corresponds to the case of a heavy oil, they lower the asphaltenes effect on viscosity. Experiments were first carried out with asphaltenes and nonylphenol as a model of resins and followed by experimental work on real crude that confirmed the trends. To understand the rheological behavior of a heavy crude oil in relation with its internal structure, asphaltenes and resins have to be simultaneously considered. From this study we conclude that the origin of the high viscosities of heavy oils comes from to the entanglement of solvated asphaltene particles stabilized by resin molecules. A strong influence of temperature on the rheology is also revealed by high activation energy measurements. This thermal dependent rheological behavior shows that the internal organization of the heavy crude oil is partly based on thermal dependent physical bonds. Using the reversibility of these physical links, we should be able to modify the structure of heavy crude oils to facilitate their transport.