Although studies have signaled a gap in trade union representation between workers with secure employment (i.e., ‘insiders’) and those without (i.e., ‘outsiders’), this gap has rarely been empirically analyzed at the micro-level. With recent micro-level data from the Netherlands, this study addresses two questions. First, to what extent do insiders and outsiders, measured through individuals’ employment status and self-perceived social risk, differ in their willingness/probability to join trade unions? Second, to what extent can these differences in trade union membership be explained as resulting from perceptions of interest representation and/or workplace social cohesion? The results suggests a clear insider–outsider gap in trade union membership related to employment status, but not to social risk. Furthermore, this gap can be explained by differences in perceptions of representation, but not workplace social cohesion.