This Elements examines the interplay between international climate change mitigation commitments and domestic policymaking, focusing on the gaps between mitigation targets in Nationally Determined Contributions (NDCs) and their national-level counterparts. It introduces a typology of countries – leading, cautious, performative, and lagging – based on their NDC ambition and policy harmonisation. By employing the novel Vertical Policy Harmonisation indices, this Elements provides theoretical, empirical, and analytical insights into the factors driving mitigation ambition and implementation gaps. Through case studies and large-N analysis, it identifies structural, political, and economic challenges affecting countries' mitigation performance. The findings underscore the need for societal transformations and robust policy frameworks to enhance mitigation ambition and harmonisation across governance levels. Concluding with actionable recommendations, the Elements contributes to a deeper understanding of how to bridge the ambition and implementation gaps critical to achieving global temperature goals. This title is also available as Open Access on Cambridge Core.