Weeds are a difficult category to define. Plants that might be considered unwanted weeds in one context could be considered desired wildflowers in another. This project questions the present day assumptions that weeds have been universally seen as "bad" plants and that weed populations were a static, unchanging issue before the advent of modern agriculture. Studying Latin and English sources from the ancient and medieval world provides new insight into how human-weed relationships changed over time in Western Europe, while synthesizing data from archaeology, history, and palaeoclimate studies poses new questions related to long-term weed biodiversity change. Chapter 1 traces attempts to define the term "weed" from the 15th century to the present day, arguing ultimately for a definition of "unintended plants". In Chapter 2, I argue that a more relational attitude towards weeds that focused on their role in agroecosystems existed in antiquity and the early medieval period before being gradually replaced by a specifically Christian point of view in which weeds were seen as punishment for and a reminder of original sin. The second half of the dissertation turns to the contextualization and study of weeds in agroecosystems in northern Britain throughout the medieval period. Chapter 3 synthesizes palaeoclimate studies with archaeological and historical narratives related to political change and agricultural transformation to identify questions about these changing systems that I attempt to answer in Chapter 4. In this final chapter, I demonstrate through an archaeobotanical study of plant remains from Bamburgh Castle in northern England that weed biodiversity has been declining for over a millennia. I argue this biodiversity decline is likely due to landscape simplification through the subdivision of multiple estates starting in the centuries immediately preceding the Norman Conquest of 1066. Ultimately, this project shows that human relationships to weeds have always been complex, nuanced, and multi-faceted, and understanding this long history is key for understanding biodiversity loss and barriers to implementing ecological weed control measures in the present day.