Book excerpt: Large caldera collapses represent catastrophic natural events, second only to large meteoritic impacts. In addition, some calderas are densely populated, making the risk extreme, even for moderate eruptions. Understanding caldera mechanisms, unrest and the danger of eruption is therefore a crucial challenge for Earth sciences. Several key features of caldera behaviour have yet to be fully understood. Through a combination of case studies and theoretical modelling, the following topics are addressed in this volume: the conditions required to produce and to release large volumes of magma erupted during caldera formation; how magmatic feeding systems evolve before and after a caldera has formed; the processes that limit the behaviour of precursors to eruptions; how pre-eruptive precursors can be distinguished from those that drive unrest without an eruption; and given that post-collapse eruptions may occur across a wide area, the optimum procedures for designing hazard maps and mitigation strategies.