The concept of social membership is the mainframe for this dissertation, which encompasses four independent articles that approach the boundaries of social membership from different perspectives. Empirically, the focus lies on mobilizing groups that demand an extension of rights and/or inclusion for documented and undocumented immigrants in two European immigration countries: Sweden and Spain. I have defined the processes through which mobilizing actors (immigrants themselves and diverse supporters of their cause) interact with boundary-making actors (institutional actors, policy makers), whom through their positions participate in drawing the boundaries between inclusion and exclusion, as negotiating social membership. To study these processes, I have performed 68 interviews with actors as mobilizing immigrants, activists mobilizing on behalf of immigrants, representatives of NGOs and trade unions, policy-makers and politicians. Two main types of claims appeared: undocumented migrants' rights groups mobilizing for residence permits and social rights, and documented immigrants' (and their supporters') advocacy against ethnic discrimination. Furthermore, I have included a study that reflects the tensions over social membership within immigrant communities. The gendered dimension is its main focus, as it illustrates the value conflicts over gender equality and ethnic diversity brought to the surface through the debates following so-called honour killings in Sweden, and the difficulties faced by young immigrant women mobilizing simultaneously against racism and patriarchal oppression.