The dominant narrative in the 20th century was centered on the complementarity and opposition between market and state. To deal with social and solidarity economy (SSE) origins and histories is to recover a forgotten memory. To go in this direction, it is possible to identify three periods in different continents. The first period was constituted by an associationalism based on democratic solidarity initiated by various groups (indigenous self-organizations in South America, women and African-American in North America, pioneering workers in Europe). It was multidimensional, imbricating political, social and economic questions. The second period saw the recognition of different legal statuses: those of the cooperative, the mutual society and the non-profit organization. Since the end of the 19th century, they have been the components of a social economy, defined as a set of non-capitalist organizations operating within an institutional framework based on the separation between the market economy and the welfare state, particularly during the post-World War II economic expansion (1945 – 1975). The third period links the different crises of the late 20th century with the emergence of the solidarity economy during the same period, which can also be considered as a resurgence of the associationalist movement. The origins and histories allow us to consider the significance of SSE in the 21st century and to address the conditions to concretize its transformative potential.