This thesis is the outcome of a six-year-long research, aiming at understanding how the flexibility-era South European workers unionize and engage in collective action. Its empirical material derives from the employment of a qualitative methodology techniques’ triangulation: archive research, participant observation and semi-structured interviews. I define as Precarious Workers’ Unions (PWUs) labor collectives the members of which (a) are subjected to atypical labor relations; (b) lack adequate access to the welfare state structures; (c) have developed a collective conscience of belonging to a post-Fordist labor force. The PWUs’ main characteristics put under scrutiny are: member recruitment, decision-making procedures, services offered, industrial and movement action undertaken. Determinants which I consider as having a significant impact on the above include each country’s labor legislation, formal trade union structure, social movement environment and tradition, as well as each PWU’s population make-up. A dual comparison is employed. On the one hand, similarities and differences are sought between the Italian PWUs and their Greek counterparts. On the other hand, an internal comparison is conducted between each country’s organizations, in order to locate and explain potential divergences from the national model. Despite the fact that the first unionizing initiatives in Greece and Italy were facing similar socio-economic structural conditions, their mobilization developed in a diversified way. Lately, a re-convergence between the two countries’ PWUs is to be noted: Mixed inside-outside the workplace interventions, a resurgence of mutualist practices and the inability to integrate in the formal trade union structure, combined with a relevant role in the broader social movement activities, are its main characteristics. Furthermore, as derives from the empirical data, attributing a unique class status to the expanding population of precarious workers may lead to erroneous assumptions. The precarious condition is a transversal, passing through the various social strata and is experienced in many different ways. The above is demonstrated not only by the significant impact of the PWUs’ population make-up on their organizational forms and activities, but also by the fact that, even inside organized labor entities, pre-existing inequalities are neither reversed nor dampened. Finally, the –partly eclectic, partly innovative- character of the PWUs is leading to the assumption that they are not only challenging the notion of precarity as perceived up to date, but also the very idea of what a union is and how it is supposed to operate. Whether this re-negotiation is to provide an answer to the 30-year-old “unions in crisis/union revitalization” riddle is not only a matter of the PWUs’ strategic choices. It is also dependent on the socio-economic context. Future research shall have to examine to what extent the post-2008 economic crisis acts as an accelerator of the tendencies identified, an obstacle – or a diversion, which shall lead the PWUs to new, unexplored territories.