Are the trade unions in crisis? The answer would appear to be yes. The drop in the number of members, the gradual reduction in their mobilization power in the face of recent social and employment cutbacks and the change in the production model, together with the lack of job security, seem to answer this question in the affirmative. In this work, we have focused on the crisis of the majority trade unions in Spain, a country that has been particularly affected by social and employment cutbacks as a result of the financial crisis of 2008. To that end, two more specific aspects concerning the role of trade unions have been studied, namely: 1) their role as political actors capable of exerting influence and reaching agreements with the government; and 2) their role as bureaucratic institutions which have been progressively distancing themselves from their rank and file. These aspects have been examined through the theories of Political Exchange by Alessandro Pizzorno (1978) and the Iron Law of Oligarchy by Robert Michels (1911), respectively. In order to analyse the former, we have focused on the agreements signed by the Spanish majority trade unions (CCOO and UGT), employer federations (CEOE and CEPYME) and the government from the Spanish democratic transition (1977) to the financial crisis of 2008, then onwards until the present day. Those agreements have served as paradigmatic study cases on how the political exchange referred to by Pizzorno came into being. For the analysis of the second theory, we have focused on the rift between the leaders and trade union structures from the rank and file, as well as on the change in internal organizational structure and the adoption of transparency measures by the trade unions as ways in which to bridge that rift. To that end, official documents, press articles relating to trade unions, the trade unions’ own websites and a questionnaire completed by an ex-member of CCOO have been analysed. According to the analysis performed, the Political Exchange theory is suitable for explaining the institutional role of trade unions in Spain, as political actors who negotiate with the government. This political exchange was nevertheless broken during the financial crisis of 2008 since, due to the measures imposed by the Troika, the government-driven labour reforms ceased to be subject to negotiation with the trade unions. As regards Michels’ Iron Law of Oligarchy theory, it would appear to be applicable, at least in part. The analysis shows how the current trade union crisis can be partly explained by an increasing disaffection, both on a social level and on the level of the workers themselves. Such disaffection can, in turn, be explained by the trade unions’ inherently bureaucratic structure and oligarchic functioning. Majority trade unions such as CCOO and UGT are currently huge, heavily bureaucratic macro-structures, with little leadership accountability in respect of the rank and file. The work concludes by indicating that Spanish trade unions have decided to base their legitimacy, at least partly, on their power to influence the government’s political and economic decisions, and seem to have placed less importance on their legitimation through the link with their rank and file.