While speculation has always been crucial to biography, it has often been neglected, denied or misunderstood. This edited collection brings together a group of international biographers to discuss how, and why, each uses speculation in their work; whether this is to conceptualise a project in its early stages, work with scanty or deliberately deceptive sources, or address issues associated with shy or stubborn subjects. After defining the role of speculation in biography, the volume offers a series of work-in-progress case studies that discuss the challenges biographers encounter and address in their work. In addition to defining the 'speculative spectrum' within the biographical endeavour, the collection offers a lexicon of new terms to describe different types of biographical speculation, and more deeply engage with the dynamic interplay between research, subjectivity and that which Natalie Zemon Davis dubbed 'informed imagination'. By mapping the field of speculative biography, the collection demonstrates that speculation is not only innate to biographical practice but also key to rendering the complex mystery of biographical subjects, be they human, animal or even metaphysical.