An exact reprint of the original Special Operations Research Office (SORO) publication from 1966. The study was the second product of SORO research on undergrounds and describes, on the basis of the existing empirical information and the then-current state of knowledge, the organizational, motivational, and behavioral characteristics of undergrounds in insurgent movements and relates these characteristics to the total revolutionary structure, mission, and operations. There are six parts to the study: Organization; Motivation and Behavior; Underground Administrative Operations; Underground Psychological Operations; Paramilitary Operations; Government Countermeasures. Three appendices give details on the methodological approach, offer an analysis of 24 insurgencies, and summarize World War II underground rules of clandestine behavior. This classic Unconventional Warfare study has been brought back into print by Conflict Research Group.