Much has been written about union wage bargaining. Much less has been written about union density, which has been viewed as simply the employment outcome under the wage bargain. This paper presents a new dynamic model of union density that exhibits multiple equilibria and path-dependency. The model builds upon Freeman (1998) who identified the importance of union spending on organizing and business spending on opposing unions. It emphasizes the demand for union representation which depends on wage bargaining outcomes, the state of labor law, and socio-economic factors impacting public attitudes to unions. The model is used to provide a narrative account of the historical evolution of union density in the U.S. and to identify factors important for its future evolution.