This book systematically covers the fundamental theories and experimental studies of fluid jet fragmentation. It consists of 11 chapters: Chapter 1 - Introduction and Overview, Chapter 2 - Control Equations and Boundary Conditions of Jet Flows, Chapter 3 - Linear Instability of Rayleigh-Taylor Waves on Planar Liquid Films, Chapter 4 - Linear Instability of Temporal, Spatial, and Spatiotemporal Modes on Planar Liquid Films, Chapter 5 - Nonlinear Instability of Planar Liquid Films, Chapter 6 - Linear Instability of Rayleigh-Taylor Waves on Circular Jets, Chapter 7 - Linear Instability of Temporal, Spatial, and Spatiotemporal Modes on Circular Jets, Chapter 8 - Nonlinear Instability of Circular Jets, Chapter 9 - Linear Instability of Rayleigh-Taylor Waves on Annular Liquid Films, Chapter 10 - Linear Instability of Temporal, Spatial, and Spatiotemporal Modes on Annular Liquid Films, Chapter 11 - Nonlinear Instability of Annular Liquid Films.