Organizations can be categorized according to the nature of their owners, as either public or private organizations. Among private organizations, there is a subset of “other” organizations that are heterogeneous and have different legal structures, with diverse objectives. In Spain, these “other organizations” are labeled the Third Sector and/or Social Economy. There are two basic foundations for these “other” private organizations. Both arise when clusters of people create population centers. As a result, neither appears without the other; rather, they emerge simultaneously, and they are both extraordinarily important for social cohesion and sustainable development. The concepts in question are mutual aid and third-party aid.