This thesis provides a reassessment of Barry's travels and early career up to late 1834. There is no modern analysis of Barry, and no biography has appeared since 1867. A reassessment is necessary as, although Barry was one of the most important early 19th century architects, his contribution to architecture has - especially in the 1930s-1960s - been ignored, either because his reputation has been belittled or because the scope of his work has been too daunting to master. This thesis shows the architectural foundation of Barry's career and analyses his early commissions in order to provide a basis for a more objective reassessment of him. To make a reassessment this thesis examines what has been written about Barry and utilises data which has hitherto been ignored or unknown. It makes extensive use of travel diaries and drawings by Barry and Wolfe - his associate -, Wolfe's MS. written for A. Barry's 1867 biography, Barry's letters and drawings owned by his descendants, albums of drawings by his office staff and visits to the buildings he designed together with those he and Wolfe examined. The thesis concludes that Barry achieved a broad and sophisticated knowledge of architecture through his travels; he possessed flexible architectural principles that freed him from aesthetic dogmatism; Wolfe played an important and under-estimated role particularly in Barry's early career; Barry always retained a love for Greek architecture; he was an able designer in Gothic; he could not be crudely termed a plagiarist or historical copyist; his re-introduction of Italianate was of great significance to English architecture and he was an artist architect who, as an eclectic able to design with virtuosity in many architectural modes, was not confined by one architectural style. The thesis makes a chronological analysis of the evolution of Barry's architectural principles and his executed and unexecuted schemes up to late 1834. It provides a clearer understanding of the periods when Barry first employed various architectural styles and places his finished architectural creations in the context of their evolutionary background.