To enable the transition to renewable and sustainable energy systems, Small and Medium-sized Enterprises (SMEs) can successfully commercialize new technologies, yet doing so is highly challenging. Moreover, governmental policy makers tend to support measures for early-stage ventures or university spin-offs, based on the assumption that sustainable energy technologies primarily arise from early-stage technology development. Attention has recently been shifting to SMEs, which can help accelerate the energy transition. By combining a literature review with an explorative multiple case study of 20 SMEs in the Dutch sustainable energy sector, this article identified barriers and drivers for technology commercialization by SMEs. A country-specific barrier is the large consumption of natural gas by Dutch households, which strongly inhibits the successful scale-up of new technologies. The study found several managerial, financial, technological and policy-related barriers and drivers which affect technology commercialization of sustainable energy technologies by SMEs. These barriers and drivers were further assessed in an actor-based analysis, which suggests that the various barriers and drivers arise from the interactions between policy makers, industry partners and end-users. The paper also discusses the policy implications of the barriers and drivers identified, and outlines several challenges for future research.