James Gibbs was the most successful architect working in England of his generation. This thesis looks at a period of his career that was of fundamental importance to his success, but that has received insufficient coverage in the scholarship: his formation and, particularly, his training in Rome. In the first chapter, new detail is added the early circumstances of Gibbs’s life in his native Aberdeen, and an exploration of his education there given. It then considers how he travelled and what might have motivated him to travel abroad. The chapter that follows focusses on Gibbs’s years in Rome and on his connections with English and Scottish communities there, and how he could have served them as a potential cultural agent. From this, the thesis proceeds to look at the architectural context of Rome in the early eighteenth century and at the identities of Gibbs’s architectural masters. Honing in on the most important of them, Carlo Fontana, it gives an account of Fontana’s studio, how it functioned, and who its most important members were during Gibbs’s years in the city. The fourth chapter then outlines Gibbs’s Roman training in clearer detail than before, which it does by drawing on a wide range of material relating to other architects’ experiences in the city, particularly the entries made by young students for the Concorsi Clementini competitions at the Accademia di San Luca, which was closely connected to Fontana. The succeeding chapter looks at the five or so years following Gibbs’s arrival in England in 1708 in which he sought fruitlessly to establish himself, which it terms ‘years of synthesis’ for the way in which Gibbs accommodated his Roman training to a new cultural context. In particular, it looks closely at a recently-discovered journey to Scotland made by Gibbs, during which he encountered a number of important architects there. The final chapter takes as its focus an idea for a type of building – a church in the manner of a classical temple – that Gibbs first unsuccessfully proposed in 1713, but was ultimately realised as the Church of St Martin’s-in-the-Fields, completed over a decade later. Here, the thesis juxtaposes Gibbs’s work to that of his contemporaries in England, particularly Colen Campbell, to illustrate how his approach was at variance from theirs. It argues that this can be accounted for by Gibbs’s training in Rome, and the different understanding of the concept of architectural invention he learned there. The thesis concludes by looking at the Radcliffe Library, Oxford, which was completed only five years before Gibbs died, to show how Gibbs’s training continued to influence his work even as his career reached its maturity.