Developing the solidarity in the path to transition: an essay on the socialism theory based on Marx - According to the main ideas that one may extract from Marx?s works on a transition theory, the socialism would be an intermediate stage for a superior system ? the communism ? made possible only after the complete development of the productive forces, capitalism?s ?historical mission?. This thesis represents one of Marx?s biggest controversies and history has demonstrated that the reproduction and the expansion of the capitalism have not been limited by the advance of the technique ? and that neither a transition process would emerge from a sudden revolutionary reaction. In this work, we put on discussion some aspects of Marx?s thought, his transition theory and the current process of crisis of the reproduction of the capitalist relations of production. In the conclusions, a series of experiences of self-management with the heading of solidarity economics are discussed.