Abstract: It has been known for centuries that a body in contact with a substrate will start to slide only when the lateral force exceeds the static friction force. The transition from static to dynamic friction is not completely well defined, since even when the lateral force is below the nominal static friction, a body can slowly creep forward due to thermal activation. Furthermore, recent experiments indicate that frictional sliding occurs by the nucleation of detachment fronts at the contact interface that sometimes appear well before the onset of global

 sliding, calling into question our understanding of friction. These findings suggest that the onset of slip is due to microscopic processes, ultimately due to the interactions between individual atoms lying on the surfaces in contact, propagating up txo the macroscale to yield collective sliding.
In this lecture, I will first discuss the onset of slip at the nanoscale considering thermal activated creep of a Xe monolayer under a small external lateral force. In this conditions, slip proceeds by the nucleation and growth of
domains in the commensurate interface between the film and the substrate.
The results of numerical simulations can be understood by the classical theory of nucleation which allows to estimate the activation energy for creep.
At the macro-scale, the presence and evolution of frictional precursors is ruled by the interplay between elastic interactions and the sample geometry, leading to important effects that are not accounted for in traditional friction laws. I will thus discuss a three dimensional model for frictional slip that allows to quantitatively reproduce the spatio-temporal evolution of experimentally observed frictional precursors and provide predictions for geometries that have not yet been investigated. Our results could also be relevant better to understand slip precursors on earthquakes faults.