Research in fault mechanics is aimed to understanding the processes controlling the instability conditions of faults, the characteristics of fault slip and the kind of the emitted seismic waves. Faults are represented by asperity models that have proved to be a suitable tool to reproduce the main aspects of seismic sources. Stress accumulation on asperities, asperity slip and stress transfers between asperities play a central role in the seismogenic process. Models allow to simulate the evolution of a fault during the inter-seismic intervals and the characteristics of seismic slip. The evolution of seismicity in active fault systems is determined by taking into account the interaction between faults, the aseismic slip and the rheological properties of the Earth’s lithosphere.