This book is our attempt to push boundaries of theorising around female serial killers. We argue that existing literature is either too preoccupied with the individual at the expense of social context or too keen to identify the typical female serial killer to the detriment of nuance and heterogeneity. A new approach is needed, which acknowledges that each female perpetrator of serial homicide represents a complex combination of social roles and identities embedded within an overarching institutional configuration of family, economy, polity, religion and education. As such, we propose institutional mediation, an approach which considers the mediating role of institutions in terms of the nature and extent of their influence upon individual choices and actions. We illustrate this approach with reference to the case of Mary Ann Cotton and in so doing, highlight the importance of social and cultural factors in exploring how structure and agency come together to create opportunity. We believe that institutional mediation has relevance beyond the study of female serial killers and may prove fruitful for homicide studies in general because, while it takes a particular type of individual to commit homicide, it takes a particular social context to enable homicide to occur.