Abraham Guillns theory on urban guerilla warfare marked a significant evolution from popular theories on guerrilla warfare during the late 1960s. Guillns theory developed from his personal experience during the Spanish Civil War and bore the deep influence of the popular theories of guerrilla warfare, including Mao Tse-tungs protracted popular war and Ernesto Che Guevaras foquismo theory. Guillns theory became very influential within Latin America from the mid-1960s to the early 1970s. His writings, most notably the Strategy of the Urban Guerrilla, also influenced guerrillas in the United States, Europe, Japan, and the Middle East. However, not much has been written about Guillns theory. Indeed, urban guerrilla theories in general have been discredited as a concept that has never succeeded in practice. Despite the general lack of research devoted to him, it is still worthwhile determining the strengths and weaknesses of Guillns theory on urban guerrilla warfare and to explore the implications of his theory. This is especially so when considering the forecasted trends that will shape the complex environment, such as megacities, United States forces are likely to operate within in the future. Abraham Guillns theory of urban guerrilla warfare provides a relevant blueprint for guerrilla warfare in the contemporary operating environment and understanding it more fully will become increasingly useful for counterinsurgents in the future.