The objective of this study is to describe, on the basis of existing empirical information and current state of knowledge, the organizational, motivational, and behavioral characteristics of undergrounds in insurgent movements and to relate these characteristics to the total revolutionary structure, mission, and operations. Particular emphasis has been placed on: (1) Describing underground organizations and relating them to the total insurgent organization. (2) Describing Communist uses of undergrounds and their role in Communist-dominated insurgencies. (3) Summarizing descriptive and empirical information on motives for joining, staying in, and defecting from underground organizations. (4) Describing certain underground administrative, psychological, and paramilitary operations, and noting the human factors which appear to be related to their success or failure. (5) Describing the organization and countermeasures used by governments to suppress or eliminate undergrounds.