Governments across the globe face considerable challenges to govern climate change adaptation. Integrating climate change adaptation into existing policy domains such as health, agriculture and water is crucial to ensure adequate, timely and coherent responses to manage the unavoidable risks of climate change. However, it is increasingly argued that these integration efforts are progressing far too slow, are not transformative enough, or are moving in the wrong direction. Previous studies on environmental and climate policy integration as well as public policy studies suggest that the likelihood, pace and nature of policy integration is strongly influenced and constrained by the institutional characteristics of governments, such as their constitutions, political systems or administrative routines. Other studies argue that policy integration is more determined by politics such as turf wars than institutional characteristics. So far, these studies struggle with combining the temporal and contextual dimensions; are either top-down or bottom-up orientated; are overly simplified due to large-n ambitions or impossible to generalise because they are single-n studies; or understand integration as outcome rather than process. This has hampered scientific and policy progress on policy integration. This VENI project aims to address these questions by elaborating a processual framework of policy integration. The study uses macro-level quantitative methods to analyse how institutional characteristics shape the depth, scope and pace of adaptation policy integration within a large sample of countries. This step is complemented with micro-level qualitative analysis using a mechanismic approach to understand and theorize the politics of adaptation policy integration in four selected countries. The results of this study will critically inform our theoretical understanding of policy integration in general and for adaptation in particular, inform policy debates on the integration of adaptation in existing policy domains across different countries, and develop country specific intervention strategies to accelerate, deepen and upscale policy integration.