Labor unions occupy a paradoxical position within Marxist theory. They are an essential yet limited vehicle for shaping the working class into a collective actor, as unions’ role is to manage the employment relationship, not transform it. This chapter assesses how that paradox has shaped Marxist debates surrounding trade unions. Unlike previous socialists, Marx and Engels highlighted unions’ necessity, while noting their structural limitations. Capitalism’s resilience after their deaths, combined with working class weakness and conservatism, led some Marxists to conclude that unions were irredeemable, destined to obstruct revolutionary impulses and create a conservative “labor aristocracy.” Others resisted this conclusion, focusing on the need for mass action and organic working-class leadership to ensure unions’ vitality. As the postwar “consensus” between labor and capital pushed revolution off the table, many Marxists adapted, with some abandoning the working class as a revolutionary agent. For the minority who did not, their task was not simply to denounce the bureaucratic and conservative character of existing unions, but how to rebuild dynamic working-class organizations in a context where labor and the left were separated. This task became more challenging as the postwar expansion came to a halt in the 1970s. While neoliberal restructuring and attacks on unions led some to pronounce the “death of class,” others cautioned not to confuse class recomposition with class disappearance. The challenge was how to rebuild power as workers’ traditional organizational vehicles faltered. This problem persists today, although some signs of renewed working-class dynamism are emerging.