This dissertation develops a theological ethics of media use in digital media societies. It does so by focusing on our formation and use of biological memory through as well as for external media use within our overarching pursuit of happiness.Current theological interpretations of digital media suffer from three limitations. First, while they attend fruitfully to the virtues and to the impact of devices and use-practices upon thought, emotion, and action, they say little about the cognitive habits that bridge between reason and external media use. Second, they have acquired an unbalanced focus on the communicative and social functions of media, to the occlusion of media's cognitive and formative functions. Third, their retrieval of theological sources is not normally specified by the particular role of media technologies in human life.Because our use of external media both reflects and shapes how we form and use our biological memories, we can address the first two limitations by analyzing memory as a bridge between reason and media. Since Christian theologians have long approached media as a way to form and extend memory, memory is also a fruitful focus point for consulting the tradition about the ethics and theological relevance of media use.Chapter 1 explains the importance of analyzing digital media as memory technologies by engaging psychologists and computer engineers with regard to media's precise relationship to biological memory. Chapter 2 assesses modern Catholic teaching on media use, highlighting Pope Pius XII's articulation of a cognitive paradigm and narrating the subsequent occlusion of that paradigm. Chapter 3 outlines the social and technological context for how medieval theology approached media in terms of memory. Chapter 4 discusses Hugh of St. Victor's theocentric ethic of reading, which I argue both supports and critiques recent concerns about the prospects of literacy in a digital culture. Chapter 5 examines memory's role in Thomas Aquinas's psychology; this account clarifies reason's interpretive cultivation and use of both memory and media in the human creature's search for perfection. Chapter 6 integrates the foregoing and explains the importance of cultivating a general responsibility for our multiple cognitive scaffolds