The Catholic Church has a well-documented tradition on labor and unions, rooted in the human right of association. This document excerpts passages that highlight this tradition. This document is intended to serve as a primer on this issue; it is not comprehensive. To read the complete text of a cited document, simply click on the link at the bottom of this page. 
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On Human Work (Laborem Exercens)
Pope John Paul II, 1981
All these rights [of workers], together with the need for the workers themselves to secure them, give rise to yet another right: the right of association, that is to form associations for the purpose of defending the vital interests of those employed in the various professions. These associations are called labour or trade unions. . . .
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Catholic social teaching supports the right of workers to choose whether to organize, join a union, and bargain collectively, and to exercise these rights without reprisal. . . .Workers, owners, employers, and unions should work together to create decent jobs, build a more just economy, and advance the common good. (no. 76)
Economic Justice for All
U.S. Catholic Bishops, 1986
(...) Perhaps the greatest challenge facing U.S. workers and unions today is that of developing a new vision of their role in the U.S. economy of the future. The labor movement in the United States stands at a crucial moment. The dynamism of the unions that led to their rapid growth in the middle decades of this century has been replaced by a decrease in the percentage of U.S. workers who are organized. American workers are under heavy pressures today that threaten their jobs. . . . In these difficult circumstances, guaranteeing the rights of U.S. workers calls for imaginative vision and creative new steps, not reactive or simply defensive strategies. (no. 108)
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On the Condition of Labor (Rerum Novarum)
Pope Leo XIII, 1891
The most important of all [workplace associations and organizations] are workingmen's unions. . . .
Such unions should be suited to the requirements of this our age - an age of wider education, of different habits, and of far more numerous requirements in daily life. It is gratifying to know that there are actually in existence not a few associations of this nature, consisting either of workmen alone, or of workmen and employers together, but it were greatly to be desired that they should become more numerous and more efficient. (no. 49)