Volcanic systems behave episodically and require mechanisms for magma segregation, instability and ascent. Here, we discuss the processes that promote the ascent of magma to the Earth's surface and prepare the onset of an eruption, thus acting as triggers, as well as the factors that prevent eruption. We describe the various petrological, geochemical and geophysical observations that reveal the triggering processes and events. Based on emerging concepts of magma plumbing systems, we consider the mechanisms involve…