This chapter considers the history of serialism in the United States and Canada. After exploring US-based ultramodern composers that used series in their writing and early engagement with Schoenberg’s methods, this chapter contemplates the contexts for the significantly increased interest in serialism that occurred in these countries after the Second World War. Many factors were at play in this development, including the role of serialist giants who arrived as émigrés from Europe as teachers and role models, the influence of US-originating modernist movements, the changing university scene, and the cultural politics of the Cold War. While European serialist exiles like Schoenberg and Krenek were highly influential, this influence was not always direct. Moreover, while US composers using highly systematic approaches have drawn most attention, the majority of Americans and Canadians using serial methods combined them with other musical techniques to produce highly original, individualistic musical languages.