This book comprehensively illustrates the elemental processes of vesiculation and crystallization recorded in volcanic products on the basis of the equilibrium and non-equilibrium theories. The book describes the derivation of equations and the basic physics behind them in detail. This textbook is fundamental in preparing for future volcanic hazards. The target readers are graduate students and researchers, but Parts I and IV are written to be understandable by undergraduate students as well, to inspire them to enter this field.