This book offers the first modern, critical edition of the diary of John William Polidori (1795–1821). Polidori’s diary was first published in 1911 by his nephew William Michael Rossetti, based on the expurgated version of the diary created by Polidori’s sister Charlotte. Polidori was the travelling physician for Lord Byron during the spring and summer of 1816, when Byron left England in the midst of the separation crisis and joined Percy Bysshe Shelley and Mary Godwin (later Shelley) in one of the most famous coteries in British literary history. This edition corrects some of the errors from the originally published version of the diary and provides substantially more annotations, while retaining Rossetti’s very useful notes and commentary. It supplements the diary with correspondence associated with the 1816 summer and with the controversy that erupted in 1819 following the publication of Polidori’s The Vampyre under Byron’s name. This is an essential text for students and critics of the period, and especially of the Diodati Circle. This comprehensive edition affords modern readers a more objective look at important primary source materials to allow them to develop a more informed opinion about those figures and their writings from this period.