Fragmentation and hierarchical structure in the interstellar medium.

Observational and theoretical work related to the fragmentation of interstellar structures is reviewed in detail. The essential features of cool interstellar structures related to star formation are examined, and a way in which observations can be used to quantify the structure is shown. The numerous proposed physical processes that may lead to fragmentation are reviewed, and theories for the origin of the largest-scale structures are discussed. The hierarchical nature of the observed structure of the interstellar medium, and the diversity of physical processes which may be responsible for this structure, are stressed.