International human rights instruments do not explicitly include protection of undocumented migrants, but arguments for their inclusion are made on both normative and pragmatic basis. These denizens are often prevented from accessing rights de facto due to social practices, even when they are accorded de jure rights through legislation. As a result, the overwhelming majority of migrants are faced with limited options, have little voice, and have to make a living among and as part of the precariat. After 1994, South Africa was increasingly seen as a favourable destination for migrants seeking asylum and/or economic opportunities. Migrants are perceived as serving as a reserve of labour that is highly flexible, easily exploited, and unlikely to seek legal recourse for violations of labour law or to join a trade union. This labour market effect is particularly apparent and problematic in host countries with pre-existing high unemployment rates. As official workers representatives trade unions have a major role to play in recognising and mitigating the dangers inherent in dividing workers into citizens and denizens. Trade unions themselves though are in decline, with union density rates falling largely as a result of increasing use of non-standard employment arrangements by employers. Trade unions find it extremely difficult to access and organise these atypical workers, many of whom are migrants. The research for this chapter considered official union publications as well as interviews with trade union officials in the construction sector in Cape Town to assess trade unions responsiveness to migrant rights claims. Migrants are generally located in the periphery due to their more vulnerable status, and this position in the labour market renders their claims to rights and the role of trade unions in supporting these claims more difficult but equally necessary.