Climate change has become one of the grand challenges for humankind. With the 2019 IPCC report demonstrating the urge to keep global warming below 1.5°C and the increasing public awareness triggered by grassroots movements such as Fridays For Future, in many nations, policymakers began to pledge to go carbon neutral by 2050. The energy sector is responsible for two-thirds of global carbon emissions, making its decarbonization pivotal for combating climate change. Critical to national energy transitions is the expansion of low-carbon technologies, but many of them—being novel and immature—fail to compete with established technologies on markets. Other low-carbon technologies that have matured began replacing established technologies but, with their emergence, also raised integration challenges. For example, wind and solar's inherent variability jeopardizes a reliable power systems operation and demands increasing flexibility. Another example are socio-economic challenges such as rising electricity prices, eroding business models of electric utilities, and partisan support of public policies for renewables. To support policymakers addressing these challenges and advancing energy transitions, this dissertation aims to improve our understanding of how policies can support the diffusion of low-carbon technologies and their system integration. I shed light on this question by investigating the impact of policies on actors' decision-making processes, including determinants, barriers, and technology adoption mechanisms. For this purpose, I use the research cases of the energy transitions in Switzerland and California, which allow an evaluation of individual technology diffusion and the evolving repercussions between technologies and the overarching system during energy transitions. I apply a mix of quantitative and qualitative methods, including agent-based modeling and case study research, and use multiple data sources, including archival data and expert interviews. I specify the overarching research question in four articles, each addressing a distinct gap in the literature. Article I evaluates the impact of historical and projected policies on the diffusion of residential solar power and battery storage in California and their collective impact on carbon emissions, the need for flexibility, and electricity prices. Article II extends this analysis by shedding light on the importance of an individual policy instrument (i.e., electricity pricing) for technology diffusion and integration and including the coupling between the power and transportation sectors. Article III investigates the development of one policy instrument in-depth (i.e., building energy codes), evaluating the challenges of the instrument's implementation and deriving learnings for general policy development. Article IV focuses on individual technology diffusion and compares the impact of multiple policies on three different technologies in the same contextual settings. This dissertation contributes to the literature in two ways. First, it provides insights on policy impact on technology diffusion. It shows that policy support can affect low-carbon technology diffusion in different ways: policy can enable technology diffusion, accelerate an ongoing one, pull forward the start of a diffusion, or leave it unaffected. Simultaneously, policies can adversely affect the economic attractiveness of conventional technologies when supporting low-carbon technologies. Further, this dissertation shows that while one policy instrument often affects multiple technologies, multiple policies have to be combined to impact one technology's diffusion substantially. Within such policy mixes, individual instruments typically have complementary effects, but they can also be opposing due to unintended side effects. Second, this dissertation provides insights for technology diffusion modeling. It outlines that socio-economic factors affect the decision-making of agents substantially. It also reinforces the call of literature on including actor behavior and heterogeneity. Further, this dissertation outlines that technologies often influence each other during their diffusion, particularly focal and complementary technologies. Finally, I demonstrate that models should include the link between technology diffusion and the overarching system, particularly when investigating advanced energy transitions. This dissertation, therefore, calls for multi-policy, multi-technology diffusion models that include necessary systemic feedbacks. Based on these contributions, the dissertation has implications for policymakers. In countries that are frontrunners in renewable energies, policymakers increasingly face the challenge of how and when to phase out renewables support. If they keep high support relative to the technology's underlying cost, there is the risk of windfall profits and increasing electricity prices. In contrast, if they withdraw support, technology diffusion might slow down, causing boom-and-bust cycles with negative consequences for the economy. Further, this dissertation outlines general policy design principles. Policymakers should keep the additional burden for consumers light, especially when implementing technology-specific policies as they can cause higher societal costs than technology-neutral requirements. Policymakers should also provide long-term regulatory certainty for industry and consumers to spur innovation. They should also learn from frontrunners but adapt learnings to the local context. Further, this dissertation has implications for electric utilities, particularly on the utility death spiral discussion. While even utilities in countries without public support for renewables might face the challenge of declining electricity sales, I show that the likely electrification of other sectors, particularly the massive uptake of electric vehicles with its attendant extra demand, likely mitigates or even reverses the death spiral.