What, if anything, was Thatcherism? This article aims to gauge the extent to which it is credible and useful to speak of Thatcherism in the singular. It does so by developing a series of periodisations of the policy-making process in different policy domains since the late 1970s. This allows us to assess the degree to which a common rationale-such as might be associated with Thatcherism (as an instinct or project)-is exhibited in the sequencing of policy developments in different fields. In the process we seek to establish a new and more generally applicable methodology for establishing the existence of path-shaping political projects (such as Thatcherism) by gauging their periodisability. We conclude that the label Thatcherism is, indeed, warranted, since an account couched in such terms can illuminate an analysis of policy development since the late 1970s in a range of fields and can contribute to our understanding of the timing and sequencing of policy developments in each domain. Thatcherism, in other words, can be said to have existed because Thatcherism is periodisable. Our approach leads us to develop a 'cascade' theory of public policy radicalism and spillover between issue domains.