This dissertation presents a theory of online propaganda and radicalization which highlights the interaction between communication, epistemology, and technology. The central focus is providing an analysis of online media communications, content posted on social media platforms and transmitted by automated recommendation systems, which better explains how propaganda and radicalization have adapted so well to this technological environment. First, propaganda is interpreted as a unique approach to communication which manipulates the expectations an audience has of successful communications, and not simply as (...) a collection of individual acts with characteristic properties. A major aspect to this interpretation of propaganda is an emphasis on those environmental factors and sociological conditions involved in communication, which can have a direct impact on our experience and interpretation of reality. Second, considering these aspects a detailed account of how users experience the technical machinery facilitating social media platforms is presented, highlighting the problematic epistemic states users are forced to inhabit. Finally, online radicalization is explained through this new propaganda interpretation, the user experience of the technology underpinning social media platforms, and the epistemic states forced by online automation. (shrink)