This study is the second product of SORO/CRESS research on undergrounds; it describes, on the basis of existing empirical information and 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; Administrative Operations; Psychological Operations; Paramilitary Operations; and 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. A glossary and an index are provided.