This thesis examines the career of Arthur Baldwinson (1908-1969), a Sydney-based modernist architect. It argues that Baldwinson was a central figure in the development of a modernist domestic architecture in Australia from the late 1930s until the late 1950s through his practice as well as his activist role in the development of the Australian design reform and arts organisations: the Modern Architecture Research Group (MARS); the Designers for Industry Association of Australia (DIAA); and the Contemporary Art Society (CAS). It is further argued that Baldwinson designed and built two of Sydney's first authentically modernist houses before the 1939-45 War and that his subsequent development and refinement of a regional methodology for modernism in Sydney's domestic architecture is at the centre of the later regionalist styles of the late 1950s and early 1960s currently described as the