In cities worldwide, low-carbon urban initiatives (LCUIs) are realised by pioneers that prove that climate mitigation strategies can be integrated in urban development trajectories. Practitioners and scholars reflect on the need to scale-up such initiatives in order to accelerate the transition to low-carbon cities. Yet, limited conceptual clarity exists regarding the meaning of the concept of ‘scaling-up’ and the factors driving this process. This article aims to contribute to practice and theory on low-carbon urban development by presenting a taxonomy on the concept of scaling-up. Moreover, an explanatory framework is presented consisting of factors expected to contribute to the impact and scaling-up of LCUIs. Two case studies were conducted to illustrate the explanatory framework. The studies are illustrative but suggest that the explanatory framework allows for a systematic understanding of how the impact of former initiatives can be explained, and how their scaling-up can be promoted.