Since the original publication of Undergrounds in Insurgent, Revolutionary, and Resistance Warfare in 1963, much has changed, but much remains relevant. The Internet, the globalization of media, the demise of Soviet Communism and the Cold War, and the rise of Islamic fundamentalism have all impacted the nature and functionality of undergrounds. The original study's observation, however, that for every guerilla fighter, there are from two to twenty-seven underground members is still true. Likewise, the report's main thesis--that the underground part of an insurgency is the sine qua non of all such movements--is demonstrably accurate today. This book examines the anatomy of undergrounds in various insurgencies of recent history. Our goal is to continue the groundbreaking work performed in the original study and update it with insights from the post-Cold War world. Primary source material for this book comes from the Tier I and Tier II Case Studies written as part of the Assessing Revolutionary and Insurgent Strategies project. Hence, these case studies should be used as companion documents for this study.