The friction inmechanical systems plays an increasingly prominent role. It isimportant to reduce friction in mechanical systems, especially the nonlinear friction. Because of its complicated nature, it is very difficult for a single model to account for various mechanisms of friction, and dozens offriction models have been put forward so far. With respect to thedevelopment of the friction modeling in mechanical systems, some important friction phenomena are first described in this paper, suchas Coulomb friction, viscous friction, Stribeck friction,presliding displacement, varying static force and friction lag,etc. Next, a few important friction models in common use are discussed, including six static and seven dynamic friction models. The forms, characteristics andapplicability of these models are discussed in detail. The formsand the parameter identification of the static friction models aresimple and easy, but they cannot be used to simulate the dynamiccharacteristics of friction. Comparatively, the dynamic frictionmodels can describe the friction more completely, with their complicated forms, which makes it difficult to identify related parameters.Then, the influence of friction modeling on the mechanical systemdynamics is briefly discussed as well as the applications offriction models to the control of high precision mechanicalsystems. Lastly, several problems to be further studied arepointed out. This paper provides some guidance in the selection of friction models in applications.