This introduction to the special issue on 'Advancing the understanding of social innovation in sustainability transitions' is situated at the intersection of transition studies and social innovation research. In the past years, linkages between the fields of social innovation research and transition studies have been established: while transition scholars increasingly focus on social innovation phenomena, social innovation scholars engage in better understanding the relations between social innovation and societal transformation and its impacts towards sustainability. With its eight contributions, this special issue further explicates and broadens out this intersection of both fields, with an empirical focus on insights on energy transitions. Social innovation is introduced as a sensitising concept to advance our understanding of sustainability transitions along three themes: process of change; power and politics; as well as policies and policy mixes. This introduction closes by providing future research avenues and implications for policy.