In many areas of science and technology, behaviour at the smallest scales has been shown to drive the performance of macroscopic systems. Such relationships are particularly pertinent in tribology, where key phenomena (e.g. friction and flow of lubricants) ultimately depend on atomic-scale interactions. Nonequilibrium molecular dynamics simulations can probe these scales and give unique insights into the tribological behaviour of complex molecular systems. In this thesis, several industrially important tribological systems are studied through nonequilibrium molecular dynamics simulations. Firstly, in order to ensure reliable results, potential models are compared in terms of their ability to reproduce realistic viscous behaviour of a model lubricant. These accurate models are then used to study the atomic-scale behaviour of various organic friction modifier
additives under boundary lubrication conditions. The effect of molecular structure and surface
coverage along with sliding velocity, pressure and surface roughness are investigated. The friction and wear reduction mechanisms of promising carbon nanoparticle additives are also examined. Finally, the effect of base oil molecular structure on friction and flow behaviour in the elastohydrodynamic lubrication regime is studied. The work has contributed to a more complete understanding of the atomic-scale behaviour of lubricants and additives and, in combination with experiments, has helped to explain several important macroscopic phenomena.