An intensive two-phase study of teachers' unions in three small school districts in southern New York State over a 10-year span investigates the changes within the union movement and assesses the impact of unionization on the school systems. The study discloses that teachers' organizations and the collective bargaining process are subject, over time, to internal and external constraints that restrict the scope and direction of union activity to teacher welfare issues, despite the sincere interest of leaders and members in substantive professional concerns. Internal constraints include the need for membership unity and for support from statewide labor organizations, while external constraints range from budgetary limitations to school board opposition to encroachment on their sphere of authority. These constraints may be counteracted by a strong commitment to professional goals by the union leadership, along with rank and file involvement in such issues. The impact of unionization in these school districts is generally beneficial: (1) greater teacher participation in decision-making facilitates workable solutions to problems; (2) administrators' fears about the union protecting incompetent teachers are largely imaginary; (3) although it limits administrators' flexibility, a contract clarifies rights and responsibilities; and (4) communication is enhanced among teachers, administrators, and school boards. (TE)