This book, Human Factors Considerations of Undergrounds in Insurgencies, is the second edition to the 1966 book of the same name. The first edition of this book was produced by the Special Operations Research Office (SORO) at American University in Washington, DC. SORO was established by the U.S. Army in 1956. During the 1950s through the mid-1960s, SORO social scientists and military personnel researched relevant political, cultural, social, and behavioral issues occurring within the emerging nations within Asia, Africa, and Latin America. The researchers conducted analyses, sometimes for the first time, on the effects of propaganda and psychological operations and the roles of the military in developing countries, and provided large bibliographies of unclassified materials related to counterinsurgency and unconventional warfare. The Army had a particular interest in understanding the processes of violent social change in order to be able to cope directly or indirectly through assistance and advice with revolutionary actions. In 1962, SORO published the Casebook on Insurgency and Revolutionary Warfare; in 1963, it published Undergrounds in Insurgent, Revolutionary, and Resistance Warfare; and in 1966, it published Human Factors Considerations of Undergrounds in Insurgencies-each of these publications remained in the Special Operations training curricula for subsequent generations. Most of the text in this edition is new. Some large sections of the first edition are retained verbatim, mostly in Chapter 3's study of Communist organizations and sections of Chapter 5 on recruitment and retention, but also in smaller sections of the other chapters as well. The ARIS project team preserved much of the overall structure, although not the specific chapters, and strove to answer many of the same underlying questions. Material from the first edition is used without citation. Material from other SORO studies is referenced like any other source. Intended as a complement to the second edition of Undergrounds in Insurgent, Revolutionary, and Resistance Warfare, this book delves deeper into theory and further into background materials and focuses less on operational details. The ARIS project team provides numerous chapter cross-references to the second edition of Undergrounds. They also drew heavily on the new, second edition of the Casebook; these cases are cited in the normal way. They also provide a table of contents at the beginning of every chapter to make the book more useful as a reference. The first edition of Human Factors was an important synthesis of a poorly understood topic and has proved to have some remarkable staying power, with much still relevant even in the edition's fifth and sixth decades. An update to the first edition is needed, however, simply because the world has changed profoundly since the 1960s and with it the Unconventional Warfare ecosystem.