This document discusses building alliances between stakeholders in social innovation. It notes that societal challenges are complex, local, and global, requiring multidisciplinary skills and solutions developed with those impacted. Effective alliances share knowledge, pool resources, take a systemic approach, test and learn rapidly, and act to maximize social impact. The document then outlines the roles that various actors, like social enterprises, non-profits, public organizations, companies, experts, financiers, incubators, platforms, and people, can play in social innovation ecosystems and alliances. It stresses cooperation between actors to address challenges.