Establishing and sustaining quality teaching is equally dependent on the capacity of organizations and networks at regional, district and school levels to productively engage in improvement efforts that are realized in the classroom. The "teacher workforce" evokes military and industrial images, huge numbers of workers who must be trained, deployed, and managed. Efficiency becomes the paramount virtue, and centralized, hierarchical controls seem the obvious strategy. Held up against these images, teacher unions appear at best anomalous and at worst counterproductive: unless aligned in both purpose and function with other organizations in order to ensure compliance and quality control in relation to centralized authority. This chapter draws attention to two of the major and unique contributions teacher unions have made to the quality of the teacher workforce over the past several decades. Specifically, this chapter describes unions' efforts with respect to attracting and retaining teachers and to improving the quality of teachers' professional preparation and ongoing learning.