In recent years, several nations have adopted institutional framework laws, so-called Climate Change Acts (CCAs), as means to enforce Paris-compliant mitigation pathways. A key institutional feature to ensure policy stability and compliance with CCAs has been the establishment of independent advisory bodies, tasked with advising on mitigation targets and policy instruments, as well as the, monitoring and evaluation of target attainment. These advisory bodies are endowed with a crucial role in the long-term evaluation and planning process: examining how the low-carbon transition pathways might be achieved. Calling attention to the question of how transition pathways should be conceived and approached, whether it is in 'bio-physical' (climate science), 'techno- economic' (technology assessment/economics) or 'socio-technical' (socio-technical transition field) terms. Recent studies have indicated that a socio-technical transitions is increasingly framed as a question of removing carbon energy from various practices and infrastructures, challenging the dominant techno-economic approach of emissions reductions using carbon-pricing instruments. This thesis explores this challenge, drawing upon a case study of the establishment of the Swedish Climate Policy Council, by means of analyzing the process of institutionalization and how transition pathways are (re)produced discursively through the practices of climate policy evaluation and planning. The main findings of this thesis is that a cross-party consensus behind the Swedish CCA was formed around institutionalizing a 'bio-physical’ mitigation pathway, monitored and safeguarded by the Council which could assign 'political embarrassment' to governments failing to comply with the interim and long-term GHG mitigation targets. Beyond this consensus, the institutional design of the Council is the result of discursive struggles between actor-coalitions supporting techno-economic versus socio-technical transition pathways. However, the recently formed Council has come to challenge previously dominant techno-economic practices of forecasting cost-efficient emissions reductions. This has been accomplished by introducing a novel socio-technical approach to climate policy evaluation: the backcasting of interrelated technological and institutional shifts believed to be necessary in bringing about a low-carbon transition or transformation. Nevertheless, as this socio-technical practice primarily backcasts upon a number of key technological innovations, with limited changes to current industrial patterns of production and consumption, doubts are raised if this approach is to be considered as constitutive of transformative transition pathways.