Aircraft gas turbines are characterized by thousands of mechanical joints (e.g. bolted joints, dove-tail and fir-tree joints) connecting components together. These joints introduce frictional contacts because contact interfaces suffer from relative (sliding) motion due to vibrations. Unfortunately, these frictional contacts are a major source of uncertainty for the correct prediction of the dynamic response of assembled structures: in fact, they introduce strong nonlinearities due to the nonlinear nature of the friction force. A great deal of research is currently being conducted to better understand and model the nonlinear frictional behaviour of joints. This study focuses on the underlying physics of friction and on the two major parameters used in the nonlinear friction contact models: the friction coefficient and the tangential contact stiffness.