A general model for the description of the stress-strain behavior of polymer networks is introduced. We assume that the network consists of a gas of phantom chains. Its deformation is accounted for by macroscopic constraints, which allow nonaffine behavior on the microscopic scale. The advantage of the model lies in the fact that the number of assumptions is smaller than in other models and-at the same time-the main experimental features are reproduced. Comparison to mechanical deformation experiments shows good agreement with the model.