Frictional slip is an important process underlying several natural hazard, such as avalanches, landslides and earthquakes. In this talk I will discuss the general conditions in which frictional instabilities are triggered. I will consider the two basic scenarios of intrinsic (i.e. creep) and extrinsic (i.e. mechanically induced) triggering, illustrating both cases with simple friction models. In the first part of the talk, I will discuss frictional slip triggered by thermal fluctuations under a small external lateral force. In this case, slip proceeds by the nucleation and growth of commensurate domains and the results can be understood using the classical theory of nucleation. In the second part of the talk, I will discuss the role of mechanical vibration in triggering frictional slip. I will illustrate the dynamics of repulsive particles confined between a horizontally driven top plate and a vertically oscillating bottom plate. Our numerical results show a suppression of the high dissipative stick-slip regime in a well defined range of frequencies that depends on the vibrating amplitude, the normal applied load, the system inertia and the damping constant. We propose a theoretical explanation of the numerical results and derive a phase diagram indicating the region of parameter space where friction is suppressed. Finally, I will discuss the response of the system to a vibrational pulse.