The authors develop a model of union effectiveness based on an analysis of organizational goals and use it to interpret data on 511 teachers' union locals in Pennsylvania. A factor analysis identifies five dimensions of union effectiveness in the locals: the quality or extent of (a) member participation, (b) the union's preparations for negotiations, (c) the union's involvement in political and civic activities, (d) a “union mentality” (union members' perception of the union as a political interest group representing them rather than merely an association that “meets and confers” with the school administration), and (e) the union's leadership. All of these dimensions except (b) are found to contribute significantly to overall union effectiveness; (d) and (e) are the most important components.