We present a bargaining model of union contract negotiations, in which the union decides between two threats: the union can strike, or it can continue to work under the expired contract. The model makes predictions about the level of dispute activity and the form disputes take. Strike incidence increases as the strike threat becomes more attractive, because of low unemployment or a real wage drop. We test these predictions by estimating logistic models of dispute incidence and dispute composition for negotiations from 1970 to 1989. We find support for the model's key predictions, but these associations are weaker after 1981.