Volcanic activity represents a great hazard for rising populations and international air traffic over volcanic regions. This book is a compilation of papers devoted to the monitoring of volcanic activity with seismic, acoustic, geodetic, and video observations, and the catalogues of volcanic eruptions. The volcanoes, representing the subject of these observations (Bezymianny, Etna, Karymsky, Kizimen, Sakurajima, Sheveluch, Stromboli, Popocatepetl, Purace, Volcan de Colima), are between the most active on our Earth. Special attention is applied to quantification of volcanic explosions from seismic and acoustic records and the estimation of the height of eruption plumes of Vulcanian explosions from seismic records. Monitoring volcanic eruptions and their forecasting based on the changes in the spectral characteristics of volcanic tremor, on the statistical analysis of appearance of Strombolian explosions, and on the analysis of historical eruption time series are also discussed.