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The relation between the chemical composition of bitumen and the physical properties, such as viscosity or viscoelasticity is very important for the use of bitumen as an advanced construction material. In view of the limited understanding of the exact molecular structure of the millions of different molecules constituting bitumen, many different proposals for the structure of bitumen have been published over the last 100 years. In this paper we propose that interactions between the molecules are the main determinants for the physical properties of bitumen. We discuss different types of interactions, typical for hydrocarbons, such as dispersive London interactions, polar interactions and pi–pi interactions, and how the strength of the interactions may be estimated by different methods. We also discuss one class of molecules in bitumen, defined by non-solubility, the asphaltenes, and why they contribute strongly to the thickening effect of bitumen, but we also conclude that asphaltenes are not the only molecules contributing to viscosity.