Since its inception in 2013, the ICT for Sustainability (ICT4S) research community has coalesced around a common set of research challenges that address this dilemma, focusing both on how to make ICT greener, and how to leverage the power of ICT to develop sustainable solutions. The ICT4S Summer School 2021 built bridges between related communities, kickstarted new scientific collaborations and nurtured the next generation of community members through a series of presentations by leading academics, and collaborative paper writing working groups.