The strand of Nineteenth-Century Anarchism known as Anarchist-Communism conceived of the abolition of both state and market, and their replacement by a system of free distribution of goods organized through federated communes. While briefly this was the most developed and sophisticated strand of anarchism, it suffered an eclipse in the face of both the failure of the Russian Revolution, and the rise of the essentially a-theoretical industrial syndicalism that blossomed in many countries during the early decades of the twentieth century. With the expansion of the state and capitalism after WWII new forms of contestation appeared, most notably, in terms of Anarchist Communist theory, in the United States. In the 1960s and 1970s two currents emerged which represented the first significant development in anarchist communist theory for fifty years. These were the Social Ecology of Murray Bookchin, and a current which grew up around the Detroit underground paper Fifth Estate, later known as "Anarchist Primitivism". It is these two strands that are the subject of this research. Not surprisingly these two perspectives, appearing around a decade apart, and both in the same country, dealt with many of the same issues. What is more surprising is that in virtually every area, the conclusions they arrive at are completely different. In this research I locate these two strands historically as developments of Anarchist Communist theory, and examine their theories in four key areas: The Primitive, History, Reason and Rationality, and Technology. Examination of these areas serves to define the projects themselves, as well as highlighting how they disagree. To explain why they disagree, this work uses a methodological approach suggested Quentin Skinner. Skinner argued that in order to fully understand a text in the history of ideas, it is necessary to understand the author's intention in writing it. The study therefore examines not only the texts, but also the backgrounds of the writers concerned, their aims in producing it, and their approaches to debate with other theorists and perspectives. Through a combination of textual analysis and recovering the intentions of the writers, the high levels of disagreement can be accounted for.