The monograph refines the theoretical and institutional foundations of Social Solidarity Economy and systematizes the organizational forms of its entities. Management models of social solidarity economy organizations have been justified, along with methodological approaches for shaping contemporary state policies in the sector. The resource management and multilevel financing mechanisms were formed for social solidarity economy organizations. Modern directions of social innovations and social entrepreneurship in the sector were identified. Successful social solidarity economy practices in the European Union and Ukraine were systematized, and directions for the sector's further deployment under the transition to sustainable development were substantiated.The monograph can be beneficial for professionals, government officials, social entrepreneurs, and researchers engaged in social solidarity economy studies, as well as for educators, doctoral candidates, and students of humanitarian and economic specialties.