The aim of this dissertation is to improve our understanding of the concept of critical junctures in political science. In this regard the dissertation examines the approach’s evolution to this point. It is argued here that the critical junctures approach, as it had developed up to now, has lacked rigour. Consequently, the approach is remoulded in ways that add to its rigour and applicability. Improving upon the approach will involve specifying clearly defined standards that enable the identification of levels of change in order to see if that change constitutes a critical juncture. This systemisation of standards will improve the usefulness of the critical junctures approach in identifying change both within and across countries over time. The remoulded critical junctures approach is employed in examining changes in the trade union peak organisations’ influence over public policy in a series of case studies in Ireland, Britain, the United States of America, and Sweden spanning the period 1945-2000. This is in order to see if these changes in the trade unions’ influence over public policy constituted critical junctures. This research is based on the extensive assessment of trade union documentation, the media, government and political party publications, and other policy papers, along with the extensive use of secondary source material from these countries. This dissertation argues that the remoulded critical junctures approach, as set out here, possesses a clarity that was lacking in the literature up until now. The clear criteria concerning the levels of change permits the identification of critical junctures. Thus, uncertainty surrounding the concept of critical junctures is diminished. What is more, the approach as set out here is designed to be readily applicable to any research project concerned with the issue of change.