Do labor unions offer the best protection for the worker? Liberating Labor questions the assumption that Christian social teaching unequivocally endorses all forms of trade unionism. If we consider the church’s defense of freedom of association, for example, compulsory union membership is clearly at odds with Christian teaching. This essay focuses on Catholic social teaching, since it provides the only systematic treatment of labor unions within Christian social thought. In the case of labor unions, Catholic social teaching advocates the role of labor unions in bringing about the common good in the relations between workers and employers insofar as freedom of association is at work in these relations. This essay shows that freedom of association is a principle that may be distilled from papal teaching on the subject of labor unions and, as such, the principle of freedom of association is the basis for the legitimacy of labor unions defended in the encyclicals.