Book excerpt: This book provides the first comprehensive compilation of cutting-edge research on Merapi volcano on the island of Java, Indonesia, one of the most iconic volcanoes in the world. It integrates results from both the natural (geology, petrology, geochemistry, geophysics, physical volcanology) and social sciences, and provides state-of-the-art information on volcano monitoring, the assessment of volcanic hazards, and risk mitigation measures. As one of Indonesia’s most active and dangerous volcanoes, Merapi is perhaps best known for its pyroclastic density currents, which are produced by gravitational or explosive lava dome failures (commonly referred to as Merapi-type nuées ardentes). Merapi’s eruptions have posed a persistent threat to life, property and infrastructure within the densely populated areas on the volcano’s flanks, as demonstrated most recently by catastrophic eruptions, which attracted worldwide media interest.