This thesis considers the passions of the private collector of theatrical ephemera within the context of the public theatre archive. It interrogates the formation, the function, and the significance of the collection, foregrounding the idiosyncratic relationship between the collector and their collection. The eventual, though not inevitable, transition of a theatre collection from a private house to a public archive is interrogated throughout the thesis. The research concentrates on three theatre collections that have made the transition from a private space to the public archive: the Gabrielle Enthoven Collection at the Victoria and Albert Museum; the Mander and Mitchenson Theatre Collection at the University of Bristol, and the Roy Waters Theatre Collection at Royal Holloway, University of London. Theoretical and critical approaches from the fields of theatre history and historiography, archive and museum studies, and the practice and psychology of collecting are consulted alongside practical archival research into the collections and personal papers of the collectors. The collector and the materials that he/she amasses are considered within a number of social, cultural, and historical contexts including: the motivations that govern the desire to collect; the social, professional, and financial networks occupied by the collector; the gender and sexuality of the collector; the role of anecdote in framing and narrating the collector and collection; the cultural capital embodied by the theatre collection, and the future of the collector and the collection in an increasingly digitised age. As the first researcher to work on the collection of Roy Waters, this thesis offers new perspectives on an under-researched area of theatre history. This interrogation of the theatre collector and collection will argue that the collector has the potential to intervene in the construction of theatrical histories and narratives, and that the personal relationship between collector and collection is deserving of greater recognition and consideration.