Membership participation has become a hot topic within trade unions. Focusing on the IG Metall, this article reconstructs the debate on participation in collective bargaining policy and reflects the reasons for a change of direction as far as participation is concerned. This will be followed by an analysis of collective bargaining policy, specifically the problem of tension between legitimacy and effectiveness. In contrast to previous conceptual approaches, the authors promote an analytical method that considers the various phases of collective bargaining. These include different functions taken up by actors, target groups and levels of participation. Against this background, examples of extended participation on the part of members and lay shop stewards (functionaries) will be presented, particularly considering their roles in achieving varying degrees of success during (in the phases) the development of collective bargaining goals as well as the implementation (enforcement of receivable) and acceptance of negotiated outcome. It will be demonstrated that participation is both normatively justified and functionally addressed. Furthermore, that although the initial top-down path towards the “participatory union” was partly well received by a broad number of members, at the same time such an approach had to contend with certain limitations. Extended opportunities for participation, however, exist in all phases of a collective bargaining movement, even in critical phases concerned with achieving a compromise. The effects of extended membership participation as well as the different forms of such participation remain open and dependent on the tension between influence and membership logic.